gotooltest: testing flake on Windows: "access is denied"

[myitcv]

From https://github.com/rogpeppe/go-internal/actions/runs/12123750949/job/34054672393?pr=282

--- FAIL: TestSimple (0.80s)
    --- FAIL: TestSimple/cover (13.19s)
        testscript.go:584: # The module uses testscript itself.
            # Use the checked out module, based on where the test binary ran. (7.894s)
            # First, a 'go test' run without coverage. (2.677s)
            > go test -vet=off
            [stdout]
            PASS
            2024/12/06 21:03:20 cannot delete temporary directory: remove $WORK\.tmp\testscript-main7[56](https://github.com/rogpeppe/go-internal/actions/runs/12123750949/job/34054672393?pr=282#step:4:57)463866\bin\foo.exe: Access is denied.
            exit status 2
            FAIL	test	0.933s
            
            [exit status 1]
            FAIL: testdata\cover.txt:9: unexpected go command failure
            
FAIL

Discovered whilst testing #282 at commit 04a2a25