Add -tags unbundle to read executables from filesystem

[kpcyrd]

This implements the approach suggested in #81 (comment), resolves #79 (assuming #80 also lands).

I'm not 100% sure how the CI is setup, but it should probably execute both of these now to make sure a change doesn't silently break either:

go build -o build/

go build -o build/ -tags unbundle

Thanks!