Append RUSTFLAGS in CI, don't clobber (#12249)

This fixes an issues where for release binaries we set some `RUSTFLAGS`
for some targets but that was getting clobbered by setting `-Dwarnings`.
This only affects Windows binaries right now and in theory means that
all future Windows binaries will be a bit more portable.

Author: alexcrichton

.github/actions/install-rust/action.yml

@@ -58,7 +58,7 @@ runs:
         EOF
 
         # Deny warnings on CI to keep our code warning-free as it lands in-tree.
-        echo RUSTFLAGS="-D warnings" >> "$GITHUB_ENV"
+        echo RUSTFLAGS="-D warnings $RUSTFLAGS" >> "$GITHUB_ENV"
 
         if [[ "${{ runner.os }}" = "macOS" ]]; then
           cat >> "$GITHUB_ENV" <<EOF