.appveyor.yml: Put /usr/bin first in PATH

The previous commit failed with the same errors as before:

  https://ci.appveyor.com/project/mbland/go-script-bash/build/v1.6.0.3

Upon closer inspection, both of the failing tests executed `perl`.
Indeed, the Appveyor images have Perl installed separately from Git and
available in the PATH:

  https://www.appveyor.com/docs/build-environment/#perl

This Perl is most likely doing the Unix-to-native path translation. This
change attempts to fix the tests by ensuring the MSYS2 Perl that comes
with Git for Windows will get executed instead.

.appveyor.yml

@@ -11,4 +11,5 @@ build: off
 test_script:
   - where bash
   - bash --version
-  - bash -c './go test'
+  - bash -c 'echo PATH="$PATH"'
+  - bash -c 'PATH="/usr/bin:$PATH" ./go test'