tool/gocross: fix regression detecting when gocross needs rebuild

Fix regression from #8108 (Mar 2023). Since that change, gocross has
always been rebuilt on each run of ./tool/go (gocross-wrapper.sh),
adding ~100ms.  (Well, not totally rebuilt; cmd/go's caching still
ends up working fine.)

The problem was $gocross_path was just "gocross", which isn't in my
path (and "." isn't in my $PATH, as it shouldn't be), so this line was
always evaluating to the empty string:

    gotver="$($gocross_path gocross-version 2>/dev/null || echo '')"

The ./gocross is fine because of the earlier `cd "$repo_root"`

Updates tailscale/corp#21262
Updates tailscale/corp#21263

Change-Id: I80d25446097a3bb3423490c164352f0b569add5f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tool/gocross/gocross-wrapper.sh

@@ -88,7 +88,7 @@ fi
 # case, cmd/cloner invokes go with GO111MODULE=off at some stage.
 #
 # Anyway, build gocross in a stripped down universe.
-gocross_path="gocross"
+gocross_path="./gocross"
 gocross_ok=0
 wantver="$(git rev-parse HEAD)"
 if [[ -x "$gocross_path" ]]; then