Revert "os/exec: return error when PATH lookup would use current directory"

Revert "os/exec: return error when PATH lookup would use current directory"

This reverts CL 381374.

Reason for revert: broke tests for x/sys/execabs.

Updates #43724.
Updates #43947.

Change-Id: I9eb3adb5728dead66dbd20f6afe1e7a77e2a26f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/403058
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>