[test] Use release mode in `swift-xcodegen.test`
This takes about twice as long to build in CI, but runs significantly
quicker (~22x). The exact time taken varies quite a bit between runs,
but this brings the worst-case overall run-time down from ~500s to
~200s. While here, make sure we pass `--skip-build` to the second
invocation to ensure we avoid building twice. Also disable using the
dependency cache since we want to test a clean build, and we just
want swiftpm to use the local deps.
