Bug 1669874 - Use `command -v` instead of `which` in mach. r=firefox-build-system-reviewers,andi,rstewart

The former is a POSIX shell construct, usually a builtin, and the latter
is an external program that is not guaranteed to be available.

When `which` is not available, a full ./mach build succeeds with this
change.

Differential Revision: https://phabricator.services.mozilla.com/D92864

mach

@@ -61,7 +61,7 @@ run_py() {
     # Try to run a specific Python interpreter.
     py_executable="$1"
     shift
-    if which "$py_executable" > /dev/null
+    if command -v "$py_executable" > /dev/null
     then
         exec "$py_executable" "$0" "$@"
     else