cmd/go/internal/work: decrement concurrentProcesses when action finishes
This fixes a bug where we only incremented concurrentProcesses but never
decremented it, causing us to run out of tokens and give all compiles
-c=1 after a point.
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64_c2s16-perf_vs_parent,gotip-linux-amd64_c3h88-perf_vs_parent,gotip-linux-arm64_c4ah72-perf_vs_parent,gotip-linux-arm64_c4as16-perf_vs_parent
Change-Id: I41f4c1edb77004cbc1772d6d672045946a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/734260 Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Michael Matloob <matloob@google.com>
TryBot-Bypass: Michael Matloob <matloob@google.com>