Otherwise subsequent tests won't see any modified GOROOT.

With this CL I can move my GOROOT, set GOROOT to the new location, and
the runtime tests pass. Previously the crash_tests would instead look
for the GOROOT baked into the binary, instead of the env var:

--- FAIL: TestGcSys (0.01s)
        crash_test.go:92: building source: exit status 2
                go: cannot find GOROOT directory: /home/bradfitz/go
--- FAIL: TestGCFairness (0.01s)
        crash_test.go:92: building source: exit status 2
                go: cannot find GOROOT directory: /home/bradfitz/go
--- FAIL: TestGdbPython (0.07s)
        runtime-gdb_test.go:64: building source exit status 2
                go: cannot find GOROOT directory: /home/bradfitz/go
--- FAIL: TestLargeStringConcat (0.01s)
        crash_test.go:92: building source: exit status 2
                go: cannot find GOROOT directory: /home/bradfitz/go

Update #10029

Change-Id: If91be0f04d3acdcf39a9e773a4e7905a446bc477
Reviewed-on: https://go-review.googlesource.com/10685Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>