fix: work around issue with user.Lookup not returning UnknownUserError

[benhoyt]

Per https://github.com/golang/go/issues/67912, under some circumstances (on my machine) user.Lookup isn't returning UnknownUserError when it should. So before this fix I get the following test failures:

$ go test ./internals/osutil
...
FAIL: user_test.go:44: userSuite.TestRealUser

user_test.go:71:
    c.Assert(err, check.IsNil)
... value *errors.errorString = &errors.errorString{s:"user: lookup username guy: no such file or directory"} ("user: lookup username guy: no such file or directory")
...

And:

$ go test ./internals/daemon
...
----------------------------------------------------------------------
FAIL: api_files_test.go:1128: filesSuite.TestWriteErrors

api_files_test.go:1195:
    checkFileResult(c, r.Result[4], pathUserNotFound, "generic-file-error", ".*unknown user.*")
api_files_test.go:273:
    c.Check(r.Error.Message, Matches, errorMsg)
... value string = "cannot look up user and group: user: lookup username user-not-found: no such file or directory"
... regex string = ".*unknown user.*"

api_files_test.go:1196:
    checkFileResult(c, r.Result[5], pathGroupNotFound, "generic-file-error", ".*unknown group.*")
api_files_test.go:273:
    c.Check(r.Error.Message, Matches, errorMsg)
... value string = "cannot look up user and group: user: lookup groupname group-not-found: no such file or directory"
... regex string = ".*unknown group.*"
...