x/build: os.TestUserConfigDir and os.TestUserCacheDir failing on plan9-arm with `permission denied`

[bcmills]

#!watchflakes
post <- builder == "plan9-arm" && pkg == "os" && test ~ `TestUser.*Dir` && date <= "2023-08-22"

For #57638, https://go.dev/cl/520262 added tests for a couple of previously-untested functions in the os package.
Those tests are currently failing on the plan9-arm builder with permission denied errors:
https://build.golang.org/log/6f395a2be0b4d0c7708c88cce47a863680a4eff1

The discussion on #29960 stated in no uncertain terms that os.UserConfigDir on Plan 9 should be $home/lib.

The tests verify that if the config directory can be located at all, the test can create a subdirectory within it. This is analogous to how a real program might store and read its own configuration data in a subdirectory of the config directory.

It seems likely to me that the builder is misconfigured in some way. I can think of three possible ways to resolve it:

• Perhaps $home should be set to a different directory (owned by the buildlet).
• Perhaps the permissions for the existing directory (/usr/glenda) and or buildlet shoud be adjusted so that the test has permission to write there.
• Perhaps the $home variable should be cleared entirely in the buildlet's environment, so that the test will know not to even try to write there.

(attn @millerresearch; CC @golang/plan9)

[millerresearch]

Yes, plan9-arm builders were configured in an overly-secure way which prevented the test user (glenda) writing to its own home directory. I'm surprised no test has revealed this before. As a workaround I've made writable lib and lib/cache directories under $workdir and bound these onto $home/lib and $home/lib/cache. That seems to make tests pass.

[bcmills]

Great, thank you!