* ci: Test on Windows too
Adds a Windows test run to CI.
Go setup relies on GHA for this
because Hermit doesn't yet support Windows.
* fix(mapper_windows_test): assert.NotNil => assert.True
assert.NotNil does not exist.
* filecontent mapper: Handle error from directory
If we couldn't read because the source is a directory,
override the original error message.
* fix(test): Handle platform-specific "file not found" messages
Abhinav Gupta