artifact_uploader_test fixed for Windows.
artifact_uploader_test fixed for Windows.

Previously:

    C:\pda\agent>go test ./agent -run TestCollect
    --- FAIL: TestCollect (0.01s)
        artifact_uploader_test.go:31: C:\pda\agent\agent
        --- FAIL: TestCollect/Smile.gif#01 (0.00s)
            artifact_uploader_test.go:140:
                    Error Trace:    artifact_uploader_test.go:140
                    Error:          Not equal:
                                    expected: "pda\\agent/test/fixtures/artifacts/gifs/Smile.gif"
                                    actual  : "pda/agent/test/fixtures/artifacts/gifs/Smile.gif"

                                    Diff:
                                    --- Expected
                                    +++ Actual
                                    @@ -1 +1 @@
                                    -pda\agent/test/fixtures/artifacts/gifs/Smile.gif
                                    +pda/agent/test/fixtures/artifacts/gifs/Smile.gif
                    Test:           TestCollect/Smile.gif#01
    FAIL
    FAIL    github.com/buildkite/agent/v3/agent     0.457s
    FAIL

buildkite/agent,buildkite/agent,buildkite/agent