feat: strip basedirs from Rust hash key for cross-machine cache hits

SCCACHE_BASEDIRS now normalizes cwd, CARGO_MANIFEST_DIR,
CARGO_WORKSPACE_DIR, CARGO_TARGET_TMPDIR, CARGO_MANIFEST_PATH,
CARGO_BIN_EXE_*, dep-info env var values, and the concatenated
argument string in the Rust compiler's hash key computation. This
enables cache hits when the same crate is compiled from different
absolute paths on different machines (e.g., CI runners with
different checkout roots).

strip_basedir_prefix now also matches when the value equals the
basedir minus its trailing '/', so `cwd == basedir` strips to the
empty string rather than passing through. Without this, two
machines with different checkout paths produced different hashes
even with matching basedirs -- the feature's central claim.

Author: mmastrac