Instead of requiring that cmd/api/run.go be edited upon each
release to include the next Go version number, look in $GOROOT/api
for files with the prefix go1* and use those instead to perform
API checks.

Change-Id: I5d9407f2bd368ff5e62f487cccdd245641ca9c9b
Reviewed-on: https://go-review.googlesource.com/83355Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>