version/mkversion: allow env config of oss git cache dir

Updates tailscale/corp#21262

Change-Id: I80bd880b53f6d851c15479f39fad62b25f1095f1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/mkversion/mkversion.go

@@ -336,11 +336,14 @@ type verInfo struct {
 const unknownPatchVersion = 9999999
 
 func infoFromCache(ref string, runner dirRunner) (verInfo, error) {
+	tailscaleCache := os.Getenv("TS_MKVERSION_OSS_GIT_CACHE")
+	if tailscaleCache == "" {
 		cacheDir, err := os.UserCacheDir()
 		if err != nil {
 			return verInfo{}, fmt.Errorf("Getting user cache dir: %w", err)
 		}
-	tailscaleCache := filepath.Join(cacheDir, "tailscale-oss")
+		tailscaleCache = filepath.Join(cacheDir, "tailscale-oss")
+	}
 	r := dirRunner(tailscaleCache)
 
 	if _, err := os.Stat(tailscaleCache); err != nil {