feat: Extend EXEPATH optimization to support ARM64 Windows

On Windows, the `gix_path::env::system_prefix()` function has two
strategies. The first is an optimization that doesn't always work,
where the `EXEPATH` environment variable is consulted. In Git Bash,
this variable is usually available, pointing somewhere into the
Git for Windows installation. Most commonly, but in practice not
universally, it points to the top-level directory of the Git for
Windows installation. `system_prefix()` on Windows looks for a
subdirectory of this directory that is named for one of the MSYS2
environment https://www.msys2.org/docs/environments prefixes, of
those Git for Windows has builds for. (Otherwise, it falls back to
traversing to such a location upward from what `git --exec-path`
reveals, which is often slower as `git` must sometimes be run.)

However, this `EXEPATH` optimization had only checked for the
`mingw64` and `mingw32` environment prefixes. This was ideal
before ARM64 builds of Git for Windows were released. But since
then, it is useful to allow the `clangarm64` environment prefix
as well, to allow the `EXEPATH` optimization to work on ARM64
builds of Git for Windows. This makes that change.

Author: EliahKagan

gix-path/src/env/mod.rs

@@ -155,7 +155,7 @@ where
     F: Fn(&str) -> Option<OsString>,
 {
     let root = PathBuf::from(var_os_func("EXEPATH")?);
-    for candidate in ["mingw64", "mingw32"] {
+    for candidate in ["clangarm64", "mingw64", "mingw32"] {
         let candidate = root.join(candidate);
         if candidate.is_dir() {
             return Some(candidate);