workflows/release-binaries: Limit the number of projects built for pull requests

The github hosted runners can't do a full build before the 6 hour
timeout, so build less projects to give the build a chance to finish.

We use more powerful runners for the actual release builds, so we
just need to make this change for pull requests where we use the
slower github hosted runners.

Author: tstellar

.github/workflows/release-binaries.yml

@@ -145,6 +145,11 @@ jobs:
             target_cmake_flags="$target_cmake_flags -DLLVM_RELEASE_ENABLE_PROJECTS='clang;lld;lldb;clang-tools-extra;polly;mlir'"
           fi
           target_cmake_flags="$target_cmake_flags -DBOOTSTRAP_BOOTSTRAP_DARWIN_osx_ARCHS=$arches -DBOOTSTRAP_BOOTSTRAP_DARWIN_osx_BUILTIN_ARCHS=$arches"
+
+          # For pull requests we use the github-hosted runners, which aren't
+          # powerful enough to do a full build, so build less projects so that
+          # the build has a chance to succeed.
+          target_cmake_flags="$target_cmake_flags -DLLVM_RELEASE_ENABLE_PROJECTS='clang;lld'"
         fi
 
         build_flang="true"