x/build/cmd/buildlet: removeAllIncludingReadonly doesn't remove read-only files on Plan 9

[0intro]

CL 177457 added the removeAllIncludingReadonly function to be able
to remove the read-only files and directories created by go mod.

https://github.com/golang/build/blob/c7f36d0/cmd/buildlet/buildlet.go#L1813

On Plan 9, the function returns after calling os.RemoveAll.
However, this doesn't work properly on Plan 9, since files cannot
be deleted from read-only directories.

The directories should be make writeable before being able to remove files.

@millerresearch

[millerresearch]

Removing the || runtime.GOOS == "plan9" clause in removeAllIncludingReadonly should fix this. Also maybe correct the comment on the preceding line s/our emphermal/are ephemeral/ ?

[gopherbot]

Change https://golang.org/cl/207283 mentions this issue: cmd/buildlet: fix removeAllIncludingReadonly on Plan 9