Support `target/<triple>/release` in the generated Makefile

[Stranger6667]

If I specify target explicitly in my ~/.cargo/config.toml:

[build]
target = "aarch64-apple-darwin"

then when installing a gem with Rust extension (css_inline in my case), the generated Makefile will point to target/release/<name>.dylib (on macOS in my case), however, the actual artifact is target/aarch64-apple-darwin/release/<name>.dylib. This leads to:

cp: target/release/libcss_inline.dylib: No such file or directory