Fix config file resolution memoization

[medikoo]

Memoize by instance, memoizing by service name doesn't work when test configuration changes in same process

Addresses issue reported here: #8212 (comment)

[codecov-commenter]

Codecov Report

Merging #8231 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #8231   +/-   ##
=======================================
  Coverage   88.16%   88.16%           
=======================================
  Files         248      248           
  Lines        9430     9430           
=======================================
  Hits         8314     8314           
  Misses       1116     1116           

Impacted Files	Coverage Δ
lib/plugins/interactiveCli/initializeService.js	100.00% <100.00%> (ø)
lib/utils/getServerlessConfigFile.js	94.11% <100.00%> (ø)

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