Diagnose report fails to send `mkmf.log`

[unflxw]

The diagnose report fails to send the mkmf.log. This happens because the report expects this file to be in the ext/ folder within the gem. However, something has changed, and this file is now created at a different point in the tree:

• vendor/bundle/
  • extensions/x86_64-linux-musl/3.1.0/ (replace with current target architecture and Ruby major version)
    • appsignal-3.4.14/ (replace with current AppSignal version)
      • mkmf.log
  • gems/
    • appsignal-3.4.14/
      • ext/
        • (no mkmf.log file here)

This was discovered while researching #1017 -- both of the attached diagnose reports have no mkmf.log file. The issue can be reproduced by using the sinatra-alpine test setup and checking out AppSignal 3.4.14.