Move the build artifacts out of source to decrease likelihood of ccache misses because of changed build directory prefix (rustc-1.55.0-src/ vs. rustc-1.56.0-src/) per PR 246245.
Move the build artifacts out of source to decrease likelihood of ccache misses because of changed build directory prefix (rustc-1.55.0-src/ vs. rustc-1.56.0-src/) per PR 246245.