misc/cgo/testplugin: avoid writing to GOROOT

One of the 'go build' commands executed by this test passed the '-i'
flag, which caused the 'go' command to attempt to install transitive
standard-library dependencies to GOROOT/pkg/$GOOS_$GOARCH_dynlink.

That failed if GOROOT/pkg was not writable (for example, if GOROOT was
owned by the root user, but the user running the test was not root).

As far as I can tell the '-i' flag is not necessary in this test.
Prior to the introduction of the build cache it may have been an
optimization, but now that the build cache is required the '-i' flag
only adds extra work.

Updates #30316

Change-Id: Ib60080a008c1941aa92b5bdd5a194d89fd6202aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/208120


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>