GitHub build workflow

[gdm85]

This PR requires #10 and allows an automatic GitHub workflow to build Lotus 1-2-3; both the IMG file downloads and the custom binutils are cached.

Unfortunately I could not make it work for Ubuntu 18.04 as it gives a weird error related to gzip decompression (with multiple files):

invalid compressed data--code out of range

If someone would like to experiment further it is sufficient to re-add ubuntu-18.04 to the matrix values.

[justdan96]

It looks like your error message should be fixed after https://github.com/taviso/123elf/commit/8589f75a73c29a4d579474ef445f672a570f5e4b

[gdm85]

It looks like your error message should be fixed after 8589f75

Yes we could use that for Ubuntu 18.04, if there is demand of a containerised build for it.