Drone: llvm apt installation

[sdarwin]

• apt-key is deprecated
• curl will retry on more conditions than wget
• move debugging output $DRONE_STAGE_MACHINE to as early as possible in the script

If this goes well and tests are passing, then it's worthwhile to copy over to any other repos also.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.4%. Comparing base (1bb9391) to head (0776859).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #1109   +/-   ##
=======================================
  Coverage     99.4%   99.4%           
=======================================
  Files          246     246           
  Lines        16778   16778           
  Branches      1812    1812           
=======================================
  Hits         16672   16672           
  Misses         106     106           

see 1 file with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1bb9391...0776859. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mborland]

Thanks Sam