GHA/linux: build and test LibreSSL with Fil-C curl, enable pytests

Build and cache LibreSSL locally with Fil-C and enable it in the Fil-C
job.

Also:
- disable test 776 in the Fil-C job. It fails consistently, and due to
  flakiness seen earlier its result is disabled. In this job it seems to
  be adding 1 to 9 minues to the test run step and fails consistently.
- include Fil-C version in the LibreSSL cache key to prepare for Fil-C
  ABI changes.
- GHA/linux: fully quote `tflags` values to avoid breaking YAML.

Tested and confirmed working with OpenSSL too, but ended up with
LibreSSL for faster, smaller builds.

Closes #19407

.github/workflows/linux.yml

@@ -193,13 +193,13 @@ jobs:
           - name: 'openssl torture !FTP'
             install_packages: libnghttp2-dev libssh2-1-dev libc-ares-dev
             generate: -DCURL_USE_OPENSSL=ON -DENABLE_DEBUG=ON -DENABLE_ARES=ON
-            tflags: -t --shallow=25 !FTP
+            tflags: '-t --shallow=25 !FTP'
             torture: true
 
           - name: 'openssl torture FTP'
             install_packages: libnghttp2-dev libssh2-1-dev libc-ares-dev
             generate: -DCURL_USE_OPENSSL=ON -DENABLE_DEBUG=ON -DENABLE_ARES=ON
-            tflags: -t --shallow=20 FTP
+            tflags: '-t --shallow=20 FTP'
             torture: true
 
           - name: 'openssl i686'
@@ -215,12 +215,13 @@ jobs:
           - name: '!ssl !http !smtp !imap'
             configure: --without-ssl --enable-debug --disable-http --disable-smtp --disable-imap --disable-unity
 
-          - name: 'Fil-C'
-            install_steps: filc
+          - name: 'libressl Fil-C'
+            install_steps: filc libressl-filc pytest
+            tflags: '!776'  # adds 1-9 minutes to the test run step, and fails consistently
             CC: /home/runner/filc/build/bin/filcc
             generate: >-
               -DBUILD_STATIC_LIBS=ON -DBUILD_SHARED_LIBS=OFF -DCMAKE_UNITY_BUILD=OFF
-              -DCURL_ENABLE_SSL=OFF -DCURL_USE_LIBPSL=OFF
+              -DOPENSSL_ROOT_DIR=/home/runner/libressl -DCURL_USE_LIBPSL=OFF
               -DCURL_ZLIB=OFF -DCURL_BROTLI=OFF -DCURL_ZSTD=OFF
               -DUSE_NGHTTP2=OFF -DCURL_DISABLE_LDAP=ON -DUSE_LIBIDN2=OFF -DCURL_USE_LIBSSH2=OFF
 
@@ -303,12 +304,12 @@ jobs:
           - name: 'event-based'
             install_packages: libssh-dev
             configure: --enable-debug --disable-shared --disable-threaded-resolver --with-libssh --with-openssl
-            tflags: -n --test-event '!TLS-SRP'
+            tflags: '-n --test-event !TLS-SRP'
 
           - name: 'duphandle'
             install_packages: libssh-dev
             configure: --enable-debug --disable-shared --disable-threaded-resolver --with-libssh --with-openssl
-            tflags: -n --test-duphandle '!TLS-SRP'
+            tflags: '-n --test-duphandle !TLS-SRP'
 
           - name: 'rustls valgrind'
             install_packages: libnghttp2-dev libldap-dev valgrind
@@ -413,6 +414,27 @@ jobs:
           cmake --build .
           cmake --install .
 
+      - name: 'cache libressl (filc)'
+        if: ${{ contains(matrix.build.install_steps, 'libressl-filc') }}
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
+        id: cache-libressl-filc
+        env:
+          cache-name: cache-libressl-filc
+        with:
+          path: ~/libressl
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ env.LIBRESSL_VERSION }}-${{ env.FIL_C_VERSION }}
+
+      - name: 'build libressl (filc)'
+        if: ${{ contains(matrix.build.install_steps, 'libressl-filc') && steps.cache-libressl-filc.outputs.cache-hit != 'true' }}
+        run: |
+          curl --disable --fail --silent --show-error --connect-timeout 15 --max-time 120 --retry 6 --retry-connrefused \
+            --location "https://github.com/libressl/portable/releases/download/v${LIBRESSL_VERSION}/libressl-${LIBRESSL_VERSION}.tar.gz" | tar -xz
+          cd "libressl-${LIBRESSL_VERSION}"
+          cmake -B . -G Ninja -DLIBRESSL_APPS=OFF -DLIBRESSL_TESTS=OFF -DCMAKE_INSTALL_PREFIX=/home/runner/libressl \
+            -DCMAKE_C_COMPILER=/home/runner/filc/build/bin/filcc -DENABLE_ASM=OFF
+          cmake --build .
+          cmake --install .
+
       - name: 'cache wolfssl (all)'
         if: ${{ contains(matrix.build.install_steps, 'wolfssl-all') }}
         uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0