cleanup: fix misc types in context, external_commands and finders

[shubhbapna]

part of #226

fixes

src/fromager/external_commands.py:32: error: Function is missing a return type annotation  [no-untyped-def]
src/fromager/external_commands.py:32: note: Use "-> None" if function does not return a value
src/fromager/context.py:138: error: Function is missing a return type annotation  [no-untyped-def]
src/fromager/finders.py:24: error: Function is missing a return type annotation  [no-untyped-def]
src/fromager/__main__.py:25: error: Call to untyped function "detect_network_isolation" in typed context  [no-untyped-call]