update build.rs with optional git command

[nxm]

lan-mouse>    Compiling unicode-normalization v0.1.24
lan-mouse>    Compiling wayland-scanner v0.31.5
lan-mouse> error: failed to run custom build command for `lan-mouse v0.10.0 (/build/source)`
lan-mouse> Caused by:
lan-mouse>   process didn't exit successfully: `/build/source/target/release/build/lan-mouse-3fcd86e7a392a2f6/build-script-build` (exit status: 101)
lan-mouse>   --- stderr
lan-mouse>   thread 'main' panicked at build.rs:11:10:
lan-mouse>   called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }
lan-mouse>   note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
lan-mouse> warning: build failed, waiting for other jobs to finish...
error: builder for '/nix/store/5adj7bizg9s4y0b7xh5n7264q1m71xbz-lan-mouse-0.10.0.drv' failed with exit code 101
error: 1 dependencies of derivation '/nix/store/v2mdrv08bi5233axwzp6hbwn8wp9bcjd-system-path.drv' failed to build
error: 1 dependencies of derivation '/nix/store/qpmdpp7vmpgh6p106z71dq7gh6knqchw-nixos-system-zeus-25.05.20241119.23e89b7.drv' failed to build
make: *** [Makefile:10: switch] Error 1

well, I think it is better not to assume that every computer has a git command installed