cmd/go: test failures due to ETXTBSY in toolchain execs

[bcmills]

#!watchflakes
post <- `go: exec go.*: text file busy`

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- `go: exec go.*: text file busy`

2023-10-20 19:23 linux-amd64-longtest go@566d94fe cmd/go.TestScript (log)

vcs-test.golang.org rerouted to http://127.0.0.1:37227
https://vcs-test.golang.org rerouted to https://127.0.0.1:40773
go test proxy running at GOPROXY=http://127.0.0.1:42473/mod
2023/10/20 19:54:56 http: TLS handshake error from 127.0.0.1:50764: EOF
2023/10/20 19:54:56 http: TLS handshake error from 127.0.0.1:50720: read tcp 127.0.0.1:40773->127.0.0.1:50720: read: connection reset by peer
2023/10/20 19:54:56 http: TLS handshake error from 127.0.0.1:50734: read tcp 127.0.0.1:40773->127.0.0.1:50734: read: connection reset by peer
2023/10/20 19:55:00 http: TLS handshake error from 127.0.0.1:36594: read tcp 127.0.0.1:40773->127.0.0.1:36594: read: connection reset by peer
--- FAIL: TestScript (0.06s)
    --- FAIL: TestScript/gotoolchain_path (0.58s)
        script_test.go:132: 2023-10-20T19:55:14Z
        script_test.go:134: $WORK=/workdir/tmp/cmd-go-test-1115262940/tmpdir3701356172/gotoolchain_path2677324945
        script_test.go:156: 
            # This test only checks that basic PATH lookups work.
            # The full test of toolchain version selection is in gotoolchain.txt. (0.000s)
            # Compile a fake toolchain to put in the path under various names. (0.572s)
            # GOTOOLCHAIN=go1.50.0 (0.010s)
            > env GOTOOLCHAIN=go1.50.0
            > ! go version
            [stderr]
            go: exec go1.50.0: text file busy
            [exit status 1]
            > stderr 'running go1.50.0 from PATH'
        script_test.go:156: FAIL: testdata/script/gotoolchain_path.txt:23: stderr 'running go1.50.0 from PATH': no match for `(?m)running go1.50.0 from PATH` in stderr

— watchflakes