Allow any execute bits to count as executable

[JaredNeil]

If the tools/bazel wrapper is executable by the user, but not everyone the current behavior is that bazelisk assumes it's not executable and silently pretends that the wrapper script doesn't exist, which was very confusing to me.

After this change, someone could run into the case where bazelisk thinks the user can execute the wrapper, but doesn't actually have permission. If that does happen, the error could not run Bazel: could not start Bazel: fork/exec /repo/tools/bazel: permission denied makes it pretty clear what's wrong.

This now matches how the Go standard library checks if a file is executable. https://cs.opensource.google/go/go/+/refs/tags/go1.19:src/os/exec/lp_unix.go;l=26