ci: fix excessive memory usage by preventing source-map generation

[pichlermarc]

Which problem is this PR solving?

Browser tests are running out of heap space using 8+ GB of memory. This PR applies the same fix as in the core repo, preventing source map generation by the istanbul library, but applied in the karma config instead of .nycrc. It looks like here source maps are already generated, and coverage is accurately mapped.

For a detailed description see the original PR in core open-telemetry/opentelemetry-js#3642 and the associated issue open-telemetry/opentelemetry-js#3628

Short description of the changes

• turn off source-map generation in the loader options, which passes it on to the istanbul instrumenter library.

[codecov]

Codecov Report

Merging #1419 (78cae1c) into main (e93a192) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1419   +/-   ##
=======================================
  Coverage   96.06%   96.06%           
=======================================
  Files          14       14           
  Lines         890      890           
  Branches      192      192           
=======================================
  Hits          855      855           
  Misses         35       35