Clean legacy build/install generation before each packaging

[darkdh]

fix #11939

Auditors: @bsclifton

Test Plan:
(Test on all platforms)

1. CHANNEL=beta npm run build-package
2. CHANNEL=beta npm run build-installer
3. CHANNEL=nightly npm run build-package
4. Note the size of the resources/app.asar (should be ~266 megs, not > 1GB)

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[darkdh]

++

[codecov-io]

Codecov Report

Merging #11991 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11991      +/-   ##
==========================================
+ Coverage   53.34%   53.38%   +0.04%     
==========================================
  Files         274      274              
  Lines       26040    26018      -22     
  Branches     4170     4170              
==========================================
- Hits        13891    13890       -1     
+ Misses      12149    12128      -21

Flag	Coverage Δ
#unittest	53.38% <ø> (+0.04%)	⬆️

Impacted Files	Coverage Δ
js/about/newtab.js	49.68% <0%> (-0.07%)	⬇️
js/constants/appConstants.js	100% <0%> (ø)	⬆️
app/browser/api/ledger.js	44.62% <0%> (+0.19%)	⬆️
js/actions/appActions.js	17.99% <0%> (+0.48%)	⬆️
app/browser/reducers/ledgerReducer.js	44% <0%> (+1.89%)	⬆️

[bsclifton]

Tested successfully on macOS, Windows, and Linux! 👍 Nice job on this one 😄 Because we are still doing 0.19.x builds, I set this for 0.19.x milestone so that we have clean builds

[bsclifton]

master 573a95b
0.21.x bf51642
0.20.x ff1747b
0.19.x 71257de