Only register the hermetic toolchain by default.
Only register the hermetic toolchain by default.

Registering all toolchains leads to one of them being selected randomly, which
is not what we want.  Users that need other toolchains should register them
explicitly.

phst/rules_elisp,phst/rules_elisp,phst/rules_elisp,phst/rules_elisp,phst/rules_elisp