fix(lambda-at-edge): ensure dist/sharp_node_modules empty before copying

[dphang]

Fixes: #979 - seems this directory was growing indefinitely with each publish from my machine without this fix, it was not caught in e2e tests since that doesn't deploy from a published package, but instead builds locally from scratch (so dist/sharp_node_modules would not exist on each CI run).

Will likely deprecate last few versions which may cause this issue (and unnecessarily use Lambda space)

[codecov]

Codecov Report

Merging #980 (0676e38) into master (668353f) will not change coverage.
The diff coverage is n/a.

❗ Current head 0676e38 differs from pull request most recent head e977df4. Consider uploading reports for the commit e977df4 to get more accurate results

@@           Coverage Diff           @@
##           master     #980   +/-   ##
=======================================
  Coverage   81.49%   81.49%           
=======================================
  Files          69       69           
  Lines        2534     2534           
  Branches      603      603           
=======================================
  Hits         2065     2065           
  Misses        402      402           
  Partials       67       67           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 668353f...e977df4. Read the comment docs.