Make EXE_NAME a const too

The `gix_path::env::git::EXE_NAME` static item, while more visible
than `NULL_DEVICE`, was crate-internal via `pub(super)`. So making
it a `const` instead, as done here, is not a breaking nor otherwise
user-facing change.

Author: EliahKagan

gix-path/src/env/git/mod.rs

@@ -73,9 +73,9 @@ where
 }
 
 #[cfg(windows)]
-pub(super) static EXE_NAME: &str = "git.exe";
+pub(super) const EXE_NAME: &str = "git.exe";
 #[cfg(not(windows))]
-pub(super) static EXE_NAME: &str = "git";
+pub(super) const EXE_NAME: &str = "git";
 
 /// Invoke the git executable to obtain the origin configuration, which is cached and returned.
 ///