Follow-up to #2604, fixes a breaking change in v1.2.0-rc0

Note that this toolchain_type became unused in that PR. We leave behind
an alias to make this a non-breaking change.

Verified in a downstream repo that requires the toolchain_type to
register pre-built `protoc`:
https://github.com/aspect-build/toolchains_protoc/pull/50/files

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Co-authored-by: Richard Levasseur <rlevasseur@google.com>
(cherry picked from commit f9779ee9c0a7b6dbfc1cdeb4a6d6a3f06d6206df)