github: Set -DCMAKE_INSTALL_MESSAGE=NEVER when testing libcxx

This matches what 28851edf164a337c334755ae33fd58f03cffd5a2 in
llvm-project did.

When setting up the built libcxx files for testing, the built
files are installed into a temporary test root, and the install of
them litters the build log with mostly uninteresting output.

Signed-off-by: Martin Storsjö <martin@martin.st>

.github/workflows/build.yml

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 37daa7d..80a4567 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -214,6 +214,7 @@
             -DLIBUNWIND_USE_COMPILER_RT=YES `
             -DLIBCXXABI_USE_LLVM_UNWINDER=YES `
             -DLIBCXX_EXTRA_SITE_DEFINES="__USE_MINGW_ANSI_STDIO=1" `
+            -DCMAKE_INSTALL_MESSAGE=NEVER `
             -DLLVM_LIT_ARGS="-v --time-tests"
           ninja
           ninja check-cxx check-cxxabi check-unwind