ci: cache cargo registry & build artifacts

[japaric]

time savings on cache hit

Job	before	after	after/before
ubuntu-stable	105s	37s	35%
macos-stable	132s	68s	51%
windows-stable	176s	98s	55%
ubuntu-nightly	92s	40s	43%
macos-nightly	127s	65s	51%
windows-nightly	163s	103s	63%

don't ask me how to replicate the caching steps into the non-matrix job without copy-pasting the whole thing because I don't know :P

[Urhengulas]

Good job!

bors r+

[bors[bot]]

Build succeeded:

• ci