Bug 1677914 - Don't require the SHELL variable to be set. r=firefox-build-system-reviewers,mhentges

And don't set SHELL on mac workers (added in da452e43b5d5 because of the
exception thrown by this code not having a fallback)

Differential Revision: https://phabricator.services.mozilla.com/D97686

python/mach/mach/mixin/process.py

@@ -27,6 +27,9 @@ elif 'MOZILLABUILD' in os.environ:
     _current_shell = os.environ['MOZILLABUILD'] + '/msys/bin/sh.exe'
 elif 'COMSPEC' in os.environ:
     _current_shell = os.environ['COMSPEC']
+elif sys.platform != "win32":
+    # Fall back to a standard shell.
+    _current_shell = "/bin/sh"
 else:
     raise Exception('Could not detect environment shell!')