PR that switched WT testing to the V5 mongoDB toolchain brought some changes to the compatibility tests script. Before the PR these tests used `mongodbtoolchain_v4_gcc.cmake` but PR changed it to `
mongodbtoolchain_stable_gcc` (see changes here). But later it turned out that not all the branches that are used by the compatibility testing contain this file since it was created only 2 years ago (e.g. mongodb-6.0.2).

 
That wasn't detected before (or right after) the merge of the V5 switch PR since a part of the compatibility testing (compatibility-test-for-patch-releases evergreen job) runs tests for randomly picked branches so it just didn't pick a branch without this file for some time.
 
The proper fix for now is to check whether stable toolchain file exists and provide v4 toolchain file instead if it's not. It's worth checking the case explicitly to catch a branch that doesn't have stable toolchain file.

Auto-generated section:

compatibility-test-for-patch-releases on compatibility-tests

Host: i-03dba52759e9293c9
Project: wiredtiger
Commit: 858144db
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Task Logs:

• task compatibility-test-for-patch-releases