cmd/tsconnect: allow root directory to be passed in

#7339 changed the root directory logic to find the ancestor of the cwd
with a go.mod file. This works when running the the binary from this
repo directly, but breaks when we're a dependency in another repo.

Allow the directory to be passed in via a -rootdir flag (the repo that
depends on it can then use `go list -m -f '{{.Dir}}' tailscale.com`
or similar to pass in the value).

Updates tailscale/corp#10165

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/common.go

@@ -71,6 +71,9 @@ func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
 }
 
 func findRepoRoot() (string, error) {
+	if *rootDir != "" {
+		return *rootDir, nil
+	}
 	cwd, err := os.Getwd()
 	if err != nil {
 		return "", err