tools: make doctool work if no internet available
Allow doctool to fallback to use local files if not building a release build.
Fixes: https://github.com/nodejs/node/issues/29918
cc @joyeecheung
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines