Allow bootstrapping without a key. Fixes rust-lang#36548
This will make it easier for packagers to bootstrap rustc when they happen
to have a bootstrap compiler with a slightly different version number.
It's not ok for anything other than the build system to set this environment variable.
r? @alexcrichton

Rollup of 23 pull requests
- Successful merges: #36964, #37108, #37117, #37124, #37161, #37176, #37182, #37193, #37198, #37202, #37208, #37218, #37221, #37224, #37230, #37231, #37233, #37236, #37240, #37254, #37257, #37265, #37267
- Failed merges: #37213, #37220, #37261

A new point-release shouldn't change any language semantics, so a local
stage0 that matches MAJOR.MINOR version should still be considered a
local-rebuild as far as `--cfg stageN` features go.
e.g. `1.14.0` should be considered a local-rebuild for any `1.14.X`.
(Bootstrap keys used to be an issue too, until rust-lang#37265.)

Detect local-rebuild by just the MAJOR.MINOR version
A new point-release shouldn't change any language semantics, so a local
stage0 that matches MAJOR.MINOR version should still be considered a
local-rebuild as far as `--cfg stageN` features go.
e.g. `1.14.0` should be considered a local-rebuild for any `1.14.X`.
(Bootstrap keys used to be an issue too, until #37265.)