make with SNAPSHOT_FILE should always use the new snapshot

[eholk]

Right now we have to do make clean first to pick up a custom snapshot. It'd be nice if make detected the SNAPSHOT_FILE variable and treated it the same as if snapshots.txt changed.

[jbclements]

Ah, me. The first bug I've been assigned to look at. Is the current behavior really "wrong"?

[catamorphism]

(bug triage) Still a bug as far as I know, but doesn't seem like a blocker.

[pnkfelix]

part of #8058

[alexcrichton]

Closing due to a lack of activity. I don't think that this is that much of an issue any more. It's pretty rare to run around with custom snapshots, and snapshots are easy enough to make these days that this probably just isn't worth it.