x/build: run both root and non-root Linux builders

[bradfitz]

We should run at least non-root Linux builder to catch tests which accidentally require root.

(but we shouldn't run all builders as non-root, else we'll miss things assuming non-root)

Perhaps we make some existing weird builder also have this role, like linux-386-387, to not add resources or columns. We could rename it to 386-387noroot if we wanted?

/cc @minux @adg @crawshaw

[bradfitz]

And @davecheney

[bcmills]

We should run at least [one] non-root Linux builder to catch tests which accidentally require root.

We should specifically run at least one non-root Linux TryBot, and at least one non-root longtest builder (but probably also at least one root longtest builder, since long tests may also be sensitive to whether the user is root).

CC @dmitshur @toothrot @cagedmantis

[bcmills]

Specifically, having ~all of the linux builders running as root causes us to skip (or fail to get adequate coverage for) any test that depends on making files or directories unwritable (as in #30316).

[gopherbot]

Change https://golang.org/cl/328771 mentions this issue: cmd/internal/moddeps: use a temporary directory for GOMODCACHE if needed

[gopherbot]

Change https://go.dev/cl/496155 mentions this issue: cmd/gofmt: fix a data race in TestPermissions