ci: Attempt to skip a full rustc compile on dist*

Currently when we're preparing cross-compiled compilers it can take
quite some time because we have to build the compiler itself three
different times. The first is the normal bootstrap, the second is a
second build for the build platform, and the third is the actual target
architecture compiler. The second compiler was historically built
exclusively for procedural macros, and long ago we didn't actually need
it.

This commit tries out avoiding that second compiled compiler, meaning we
only compile rustc for the build platform only once. Some local testing
shows that this is promising, but bors is of course the ultimate test!

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