Need a bootstrapping solution for distros that does not hit the network

[brson]

As mentioned in this PR, some distros appear to be bootstrapping not by using the --enable-local-rust flag to find the bootstrap compiler but by using rustbuild's cached stage0 compiler. To do this though they have to patch the build system so that it doesn't hit the network - rustbuild isn't designed for this use case. There is probably a simple solution we can come up with for this but so far haven't found the motivation to do so. I tried to summarize the situation here.

[infinity0]

Just do what @hasufell does in #36069 except guard it by a configure option that hits the network by default, --assume-local-hashes-are-fresh or something along those lines. I don't know how rustbuild works though, this wasn't discussed much in that thread.

[cuviper]

FWIW, I was doing similar in Fedora, using --enable-local-rust when I want /usr/bin/rustc, but otherwise just dropping snapshots into dl/ and patching the download scripts to accept that. Cargo needed similar manipulation.

For 1.12.0, I'm switching to always use --enable-local-rust, either pointing to /usr or extracting the snapshot and pointing into that. Ditto Cargo's --local-cargo. It seems to work fine so far.

[alexcrichton]

Implemented