build: Be more subtle about cross compilation errors

Summarize platforms that fail to build, without overloading the build
log with errors that we anyway ignore. (Currently freebsd/riscv64 fails
to build.)

.github/workflows/build-syncthing.yaml

@@ -393,12 +393,18 @@ jobs:
             | grep -v /wasm \
           )
 
+          # Build for each platform with errors silenced, because we expect
+          # some oddball platforms to fail. This avoids a bunch of errors in
+          # the GitHub Actions output, instead summarizing each build
+          # failure as a warning.
           for plat in $platforms; do
             goos="${plat%/*}"
             goarch="${plat#*/}"
-            if ! go run build.go -goos "$goos" -goarch "$goarch" tar ; then
-              echo "*** $plat failed ***"
+            echo "::group ::$plat"
+            if ! go run build.go -goos "$goos" -goarch "$goarch" tar 2>/dev/null; then
+              echo "::warning ::Failed to build for $plat"
             fi
+            echo "::endgroup::"
           done
         env:
           CGO_ENABLED: "0"