cmd/printdep: add --next flag to use rc Go build hash instead

Updates tailscale/corp#36382

Change-Id: Ib7474b0aab901e98f0fe22761e26fd181650743c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

assert_ts_toolchain_match.go

@@ -17,6 +17,9 @@ func init() {
 		panic("binary built with tailscale_go build tag but failed to read build info or find tailscale.toolchain.rev in build info")
 	}
 	want := strings.TrimSpace(GoToolchainRev)
+	if os.Getenv("TS_GO_NEXT") == "1" {
+		want = strings.TrimSpace(GoToolchainNextRev)
+	}
 	if tsRev != want {
 		if os.Getenv("TS_PERMIT_TOOLCHAIN_MISMATCH") == "1" {
 			fmt.Fprintf(os.Stderr, "tailscale.toolchain.rev = %q, want %q; but ignoring due to TS_PERMIT_TOOLCHAIN_MISMATCH=1\n", tsRev, want)