chore: Clean up overcoupling

In an urgent fix I over-coupled the windows install code against
the work done by the previous author.  This removes that coupling
in favour of a #[cfg(unix)] as is used elsewhere in self_update.rs.

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

Author: kinnison

src/cli/self_update.rs

@@ -338,7 +338,10 @@ pub fn install(
 
     let install_res: Result<utils::ExitCode> = (|| {
         install_bins()?;
+
+        #[cfg(unix)]
         do_write_env_files()?;
+
         if !opts.no_modify_path {
             do_add_to_path()?;
         }

src/cli/self_update/windows.rs

@@ -144,11 +144,6 @@ pub fn do_add_to_path() -> Result<()> {
     _apply_new_path(new_path)
 }
 
-// Nothing to do for Windows for now
-pub fn do_write_env_files() -> Result<()> {
-    Ok(())
-}
-
 fn _apply_new_path(new_path: Option<String>) -> Result<()> {
     use std::ptr;
     use winapi::shared::minwindef::*;