.appveyor.yml: Fix breakage via bash -c

The following test cases failed in the initial build:

  https://ci.appveyor.com/project/mbland/go-script-bash/build/v1.6.0.1

  log/log-command: fatal status for subcommand of command in another
    language
  vars: run perl script; _GO_* variables are exported

The failures were due to `/tmp` getting expanded to
`C:/Users/appveyor/AppData/Local/Temp/1` and `/c` getting expanded to
`C:`. This is probably because running `bash ./go` directly is setting
`BASH` and other variables using the native path format, rather than the
MSYS2 Unix-like translation.

Launching the `./go` script via `bash -c` should fix this, by ensuring
`./go` launches inside a MSYS2 `bash` process that sets the paths to
the Unix-like format.

.appveyor.yml

@@ -11,4 +11,4 @@ build: off
 test_script:
   - where bash
   - bash --version
-  - bash ./go test
+  - bash -c './go test'