Add cargo-binstall metadata so prebuilt release binaries are used

[igitur]

Adds the one [package.metadata.binstall] entry needed for cargo binstall git-delta to install the prebuilt release binaries.

binstall already finds and downloads delta-<version>-<target>.tar.gz, but it renders its default pkg-url templates with the binary name (delta) while inferring the archive directory with the crate name (git-delta). It therefore looks for git-delta-0.20.1-x86_64-unknown-linux-gnu/delta, doesn't find it, reports "please contact with upstream to fix this issue", and falls back to compiling from source. Naming the directory explicitly fixes that. No CI or release-asset changes are needed.

Deliberately no pkg-fmt: leaving it unset makes binstall try every supported format, which is what lets the Windows .zip work with this same entry.

Refs #1587 (the issue carries the design discussion, so this is intentionally not a Closes).

Verification — cargo-binstall 1.24.0, --strategies crate-meta-data --dry-run, run against this repo's own Cargo.toml via --manifest-path:

• unpatched: ERROR resolve: When resolving git-delta bin delta is not found … please contact with upstream, then falls back to installing from source.
• patched: downloads the official delta-0.20.1-x86_64-unknown-linux-gnu.tar.gz and resolves delta for install.

The same metadata also resolves x86_64-unknown-linux-musl, x86_64-pc-windows-msvc (from the .zip) and aarch64-apple-darwin in dry runs. x86_64-apple-darwin has no published artifact, so it still falls back to source — that's an artifact gap, not a metadata one.

The investigation and the dry-run verification were done with AI assistance; I've reviewed the change.