GHA/linux: build and enable nghttp2 for Fil-C job

pytests after: 527 passed, 286 skipped
pytests before: 392 passed, 423 skipped

runtests after: TESTDONE: 1646 tests out of 1646 reported OK: 100%
runtests before: TESTDONE: 1643 tests out of 1643 reported OK: 100%

Ref: b81d30ade3 #19458

Closes #19457

.github/workflows/linux.yml

@@ -55,6 +55,8 @@ env:
   RUSTLS_VERSION: 0.15.0
   # handled in renovate.json
   OPENLDAP_VERSION: 2.6.10
+  # renovate: datasource=github-tags depName=nghttp2/nghttp2 versioning=semver registryUrl=https://github.com
+  NGHTTP2_VERSION: 1.68.0
   # renovate: datasource=github-tags depName=pizlonator/fil-c versioning=semver-partial registryUrl=https://github.com
   FIL_C_VERSION: 0.674
 
@@ -216,14 +218,15 @@ jobs:
             configure: --without-ssl --enable-debug --disable-http --disable-smtp --disable-imap --disable-unity
 
           - name: 'libressl Fil-C'
-            install_steps: filc libressl-filc pytest
+            install_steps: filc libressl-filc nghttp2-filc pytest
             tflags: '!776'  # adds 1-9 minutes to the test run step, and fails consistently
             CC: /home/runner/filc/build/bin/filcc
+            PKG_CONFIG_PATH: /home/runner/nghttp2/lib/pkgconfig
             generate: >-
               -DBUILD_STATIC_LIBS=ON -DBUILD_SHARED_LIBS=OFF -DCMAKE_UNITY_BUILD=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
+              -DCURL_DISABLE_LDAP=ON -DUSE_LIBIDN2=OFF -DCURL_USE_LIBSSH2=OFF
 
           - name: 'clang-tidy'
             install_packages: clang-tidy libssl-dev libidn2-dev libssh2-1-dev libnghttp2-dev libldap-dev libkrb5-dev librtmp-dev libgnutls28-dev
@@ -435,6 +438,27 @@ jobs:
           cmake --build .
           cmake --install .
 
+      - name: 'cache nghttp2 (filc)'
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
+        id: cache-nghttp2-filc
+        env:
+          cache-name: cache-nghttp2-filc
+        with:
+          path: ~/nghttp2
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ env.NGHTTP2_VERSION }}-${{ env.FIL_C_VERSION }}
+
+      - name: 'build nghttp2 (filc)'
+        if: ${{ contains(matrix.build.install_steps, 'nghttp2-filc') && steps.cache-nghttp2-filc.outputs.cache-hit != 'true' }}
+        run: |
+          curl --disable --fail --silent --show-error --connect-timeout 15 --max-time 120 --retry 6 --retry-connrefused \
+            --location "https://github.com/nghttp2/nghttp2/releases/download/v${NGHTTP2_VERSION}/nghttp2-${NGHTTP2_VERSION}.tar.xz" | tar -xJ
+          cd "nghttp2-${NGHTTP2_VERSION}"
+          cmake -B . -G Ninja -DENABLE_LIB_ONLY=ON -DBUILD_TESTING=OFF -DENABLE_DOC=OFF -DCMAKE_INSTALL_PREFIX=/home/runner/nghttp2 \
+            -DBUILD_STATIC_LIBS=ON -DBUILD_SHARED_LIBS=OFF \
+            -DCMAKE_C_COMPILER=/home/runner/filc/build/bin/filcc
+          cmake --build .
+          cmake --install .
+
       - name: 'cache wolfssl (all)'
         if: ${{ contains(matrix.build.install_steps, 'wolfssl-all') }}
         uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0