.github/workflows/build.yml: add "mold" build

Author: MaxKellermann

.github/workflows/build.yml

@@ -29,18 +29,21 @@ jobs:
             os: ubuntu-24.04
             cc: gcc-14
             cxx: g++-14
-            packages: g++-14 libfmt-dev
+            ldflags: -fuse-ld=mold
+            packages: g++-14 mold libfmt-dev
             meson_options:
           - compiler: gcc11
             os: ubuntu-22.04
             cc: gcc-11
             cxx: g++-11
+            ldflags:
             packages: g++-11
             meson_options: --force-fallback-for=fmt
           - compiler: gcc10
             os: ubuntu-22.04
             cc: gcc-10
             cxx: g++-10
+            ldflags:
             packages: g++-10
             meson_options: --force-fallback-for=fmt
 
@@ -49,6 +52,7 @@ jobs:
     env:
       CC: ccache ${{ matrix.cc }}
       CXX: ccache ${{ matrix.cxx }}
+      LDFLAGS: ${{ matrix.ldflags }}
 
     steps:
       - id: checkout