ci: limit E2E tests to at most 2 concurrent jobs across runs

[Pijukatel]

Run at most 2 E2E test jobs at a time across all workflow runs to avoid hitting Apify API rate limits.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.75%. Comparing base (676870a) to head (b7be714).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1145      +/-   ##
==========================================
- Coverage   92.78%   92.75%   -0.03%     
==========================================
  Files          53       53              
  Lines        3519     3519              
==========================================
- Hits         3265     3264       -1     
- Misses        254      255       +1     

Flag	Coverage Δ
e2e	32.73% <ø> (ø)
integration	53.33% <ø> (-0.03%)	⬇️
unit	89.34% <ø> (ø)

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[vdusek]

LGTM; if this is gonna work, could you do the same for integration tests of the API client?

[Pijukatel]

LGTM; if this is gonna work, could you do the same for integration tests of the API client?

Lets wait to see if we are happy with the behavior and apply it in the client as well if yes.