ci(circleci): key the toolchain cache off a file that always exists

Restore/Save Toolchain Cache have been failing on every job whose toolchain
is hosted on GitHub - arm-gcc, arm-clang, riscv-gcc, rx-gcc, ft9xx-gcc:

  Restore Toolchain Cache
  template: cacheKey:1:8: executing "cacheKey" at <checksum "toolchain_key">:
  error calling checksum: open /home/circleci/project/tinyusb/toolchain_key:
  no such file or directory

"Set toolchain url and key" only wrote toolchain_key when the URL was not a
github.com link, but both cache steps referenced {{ checksum "toolchain_key" }}
unconditionally, so for those toolchains the key could never be computed. The
job still went green because the build step does not depend on the cache, which
is why this went unnoticed - but the two steps are permanently red and the
toolchain is re-downloaded on every single run.

Key the cache on the toolchain name plus a checksum of toolchain.json instead.
That file is in the repo, so the checksum always resolves, and the key still
invalidates whenever a toolchain URL changes. toolchain_key is no longer needed.

Side effect worth calling out: GitHub-hosted toolchains are now cached rather
than skipped. That was the intent of the removed condition, but it is also what
broke the steps - CircleCI cannot skip a cache step on a value only known at
run time. Caching them also saves the repeated download.

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