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>

version-embed.go

@@ -26,6 +26,12 @@ var AlpineDockerTag string
 //go:embed go.toolchain.rev
 var GoToolchainRev string
 
+// GoToolchainNextRev is like GoToolchainRev, but when using the
+// "go.toolchain.next.rev" when TS_GO_NEXT=1 is set in the environment.
+//
+//go:embed go.toolchain.next.rev
+var GoToolchainNextRev string
+
 //lint:ignore U1000 used by tests + assert_ts_toolchain_match.go w/ right build tags
 func tailscaleToolchainRev() (gitHash string, ok bool) {
 	bi, ok := debug.ReadBuildInfo()