Mark debug builds in skip --help and skip doctor

[dfabulich]

Before, when running my own hand-built Skip:

% ./skip --help | grep OVERVIEW
OVERVIEW: skip 1.7.0
% PATH=.:$PATH ./skip doctor | grep "Skip version"
[✓] Skip version 1.7.0 (= 1.7.0)

% ./skip --help | grep OVERVIEW
OVERVIEW: skip 1.7.0 (debug)
% PATH=.:$PATH ./skip doctor | grep "Skip version"
[!] Skip version 1.7.0 (debug)

(I think there might be a better way to mark official builds with isOfficial or something? But this is good for a start.)

Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device

[marcprux]

I'm confused by this. If we are running a local debug build of skip, don't we want the version that it forks to also be the same debug build? Why is it a .warn scenario when the forked version contains "debug"?