subsecond/cli: fix linker and passing through -fuse-ld properly

[laundmo]

in LinkAction, dunce::canonicalize(linker) was being called, which resulted in a "file not found" IoError as it tried to find the linker relative to the workdir, even though linker="command_on_PATH" is accepted by Cargo. This falls back to using the linker command directly in those cases.

This used to only pass through -Wl,-fuse-ld=mold when thin linking, but Rustflags uses -C link-arg=-fuse-ld=mold which causes -fuse-ld=mold to be passed. This PR adds that case.

These changes present a big annoyance with setting up subsecond to use a linker besides the default, like Mold. With this PR the following .cargo/config.toml works as expected:

[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = [
    "-C",
    "link-arg=-fuse-ld=mold",
]

Closes:

• Subsecond fails when linker is specified #4193