feat(tools/magrathea): let users override the repository root

for global installations of magrathea, setting MG_ROOT can be a way to
switch quickly between different repositories (esp. in combination
with `cd (mg path)`).

Change-Id: I4627fe78b7cc112b75ab57e7806ffd85c6d38aee
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4396
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

tools/magrathea/mg.scm

@@ -168,9 +168,10 @@ USAGE
   (or mg--repository-root
       (begin
         (set! mg--repository-root
+              (or (get-environment-variable "MG_ROOT")
                   (string-chomp
                    (call-with-input-pipe "git rev-parse --show-toplevel"
-                                     (lambda (p) (read-string #f p)))))
+                                         (lambda (p) (read-string #f p))))))
         mg--repository-root)))
 
 ;; determine the current path relative to the root of the repository