Improve message when repo submods not initialized

[justsmth]

Description of changes:

Improve messaging when a contributor clones our repo but doesn't initialize the submodules prior to a build.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[WillChilds-Klein]

giving the user the proper command to run might be a minor upgrade here. ACCP does this.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.89%. Comparing base (c358484) to head (8acd16d).
Report is 145 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
- Coverage   95.80%   92.89%   -2.91%     
==========================================
  Files          61       67       +6     
  Lines        8143     9718    +1575     
  Branches        0     9718    +9718     
==========================================
+ Hits         7801     9028    +1227     
- Misses        342      402      +60     
- Partials        0      288     +288     

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