GHA: enable H3 proxy in CI, also enable h2o tests on Linux

Also:
- GHA/http3-linux: enable deprecated APIs in openssl-prev local
  OpenSSL builds. Required by h2o and its vendored dependencies.
  Tried OpenSSL 4, LibreSSL 4.x, BoringSSL: all failed at one point.
- GHA/http3-linux: build h2o from source.
  libuv1-dev may not be stricly required.
  Tried installing libwslay-dev, but it wasn't recognized.
  Also disable building h2o libs for a much smaller dist directory and
  slightly faster build.
  Sadly, h2o is not versioned, so I pinned to the current latest commit
  at the master branch. It advertises itself as 2.3.0-DEV in pytest.
- drop redundant `libnghttp3` installs. Remains of openssl-quic builds.
  Follow-up to 6aaac9dd38 #20226

Note GHA/macos pytests may or not not be stable with the H3 proxy tests.

Follow-up to e78b1b3ecc #21153

Closes #21789

.github/workflows/macos.yml

@@ -244,7 +244,7 @@ jobs:
 
           - name: 'OpenSSL libssh'
             compiler: llvm@18
-            install: libssh libnghttp3
+            install: libssh
             generate: -DENABLE_DEBUG=ON -DCURL_USE_LIBSSH2=OFF -DCURL_USE_LIBSSH=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DCURL_BROTLI=OFF -DCURL_ZSTD=OFF
 
           - name: '!ssl HTTP-only c-ares'
@@ -275,13 +275,13 @@ jobs:
             install_steps: pytest
             generate: >-
               -DENABLE_DEBUG=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DUSE_NGTCP2=ON -DCURL_BROTLI=OFF -DCURL_ZSTD=OFF -DCURL_USE_LIBSSH2=OFF
-              -DCMAKE_C_STANDARD=90 -DCURL_ENABLE_NTLM=ON
+              -DCMAKE_C_STANDARD=90 -DCURL_ENABLE_NTLM=ON -DUSE_PROXY_HTTP3=ON
 
           - name: 'OpenSSL SecTrust'
             compiler: clang
             install: libnghttp3 libngtcp2
             install_steps: pytest
-            configure: --enable-debug --with-openssl=/opt/homebrew/opt/openssl --with-ngtcp2 --with-apple-sectrust --enable-ntlm
+            configure: --enable-debug --with-openssl=/opt/homebrew/opt/openssl --with-ngtcp2 --with-apple-sectrust --enable-ntlm --enable-proxy-http3
 
           - name: 'OpenSSL event-based'
             compiler: clang
@@ -293,7 +293,7 @@ jobs:
             install: openssl@4 libnghttp3 libngtcp2 gsasl
             generate: >-
               -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl@4 -DUSE_ECH=ON -DCURL_USE_GSASL=ON -DUSE_APPLE_IDN=ON -DUSE_NGTCP2=ON -DCURL_DISABLE_VERBOSE_STRINGS=ON
-              -DUSE_APPLE_SECTRUST=ON -DCURL_ENABLE_NTLM=ON
+              -DUSE_APPLE_SECTRUST=ON -DCURL_ENABLE_NTLM=ON -DUSE_PROXY_HTTP3=ON
 
           - name: 'MultiSSL AppleIDN clang-tidy +examples'
             image: macos-26
@@ -326,7 +326,7 @@ jobs:
               -DCURL_USE_GSSAPI=ON -DGSS_ROOT_DIR=/opt/homebrew/opt/krb5
               -DCURL_BROTLI=ON -DCURL_ZSTD=ON
               -DCURL_CLANG_TIDY=ON -DCLANG_TIDY=/opt/homebrew/opt/llvm/bin/clang-tidy
-              -DCURL_ENABLE_NTLM=ON
+              -DCURL_ENABLE_NTLM=ON -DUSE_PROXY_HTTP3=ON
 
           - name: 'LibreSSL openldap krb5 c-ares +examples'
             compiler: clang
@@ -374,21 +374,21 @@ jobs:
 
           - name: 'OpenSSL torture 1'
             compiler: clang
-            install: openssl@4 libnghttp3
+            install: openssl@4
             install_steps: torture
             generate: -DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DENABLE_THREADED_RESOLVER=OFF -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl@4 -DUSE_ECH=ON -DCURL_ENABLE_NTLM=ON
             tflags: '-t --shallow=25 --min=480 1 to 500'
 
           - name: 'OpenSSL torture 2'
             compiler: clang
-            install: openssl@4 libnghttp3
+            install: openssl@4
             install_steps: torture
             generate: -DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DENABLE_THREADED_RESOLVER=OFF -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl@4 -DUSE_ECH=ON -DCURL_ENABLE_NTLM=ON
             tflags: '-t --shallow=25 --min=730 501 to 1250'
 
           - name: 'OpenSSL torture 3'
             compiler: clang
-            install: openssl@4 libnghttp3
+            install: openssl@4
             install_steps: torture
             generate: -DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DENABLE_THREADED_RESOLVER=OFF -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl@4 -DUSE_ECH=ON -DCURL_ENABLE_NTLM=ON
             tflags: '-t --shallow=25 --min=628 1251 to 9999'