Add BUILD_VERSION to lib.rs

In CMake this used a `version` file in the CARGO_MANIFEST_DIR, but
relying on that is problematic due to change-detection, as if we add
`cargo-rerun-if-changed:version`, cargo would rerun every time if the file does
not exist, since cargo would expect the file to be generated by the
build-script. We could generate it, but that relies on the output of `git
describe`, whose dependencies we can only limit to anything in the
`.git`-folder, again causing unnecessary build-script runs.

Instead, this reads the `FISH_BUILD_VERSION`-env-variable at compile time
instead of the `version`-file, and falls back to calling git-describe through
the `git_version`-proc-macro. We thus do not need to deal with extraneous
build-script running.

Cargo.toml

@@ -60,6 +60,7 @@ rand = { version = "0.8.5", features = ["small_rng"] }
 unixstring = "0.2.7"
 widestring = "1.0.2"
 rand_pcg = "0.3.1"
+git-version = "0.3"
 
 [build-dependencies]
 autocxx-build = "0.23.1"
@@ -80,4 +81,4 @@ fish-ffi-tests = ["inventory"]
 
 # The following features are auto-detected by the build-script and should not be enabled manually.
 asan = []
-bsd = []
+bsd = []