tool/gocross: make gocross-wrapper use GOCROSS_WANTVER if set instead of git

To run tests in an environment without a git checkout and just the bare
files (from git archive)

Updates #9841

Change-Id: Ibaf5e230cd8e6af7b25540b773df54d3e1ac25e1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tool/gocross/gocross-wrapper.sh

@@ -90,7 +90,10 @@ fi
 # Anyway, build gocross in a stripped down universe.
 gocross_path="gocross"
 gocross_ok=0
-wantver="$(git rev-parse HEAD)"
+wantver="${GOCROSS_WANTVER:-}"
+if [[ "$wantver" == "" ]]; then
+    wantver="$(git rev-parse HEAD)"
+fi
 if [[ -x "$gocross_path" ]]; then
 	gotver="$($gocross_path gocross-version 2>/dev/null || echo '')"
 	if [[ "$gotver" == "$wantver" ]]; then

tool/gocross/toolchain.go

@@ -15,6 +15,9 @@ import (
 )
 
 func toolchainRev() (string, error) {
+	if v := os.Getenv("GO_TOOLCHAIN_REV"); v != "" {
+		return v, nil
+	}
 	// gocross gets built in the root of the repo that has toolchain
 	// information, so we can use os.Args[0] to locate toolchain info.
 	//