interp: "text file busy" test flakes

[mvdan]

 --- FAIL: TestRunnerRun (0.02s)
    --- FAIL: TestRunnerRun/880 (0.00s)
        interp_test.go:2906: wrong output in "mkdir c; echo '#!/bin/sh\necho b' >c/a; chmod 0755 c/a; c/a":
            want: "b\n"
            got:  "fork/exec /tmp/interp-test984148057/c/a: text file busy"
FAIL
FAIL	mvdan.cc/sh/v3/interp	0.526s

https://github.com/mvdan/sh/runs/3542787342?check_suite_focus=true

[mvdan]

There's a slim chance that 91c203e5cba4e73bc1752109cf283de10f11b863 fixed this. At least, I can no longer manage to reproduce locally.

[mvdan]

Nope, not fixed - I happened to run into it yet again locally.