Run debug-runtime workflows with a reduced minor heap

[jmid]

This little PR reduces the minor heap size of the debug-runtime workflows.
I'm expecting the reduced size to stress-test the GC-logic more than the default size.

The s=4096 words matches that of the ocaml/ocaml debug runtime workflow.

Finally, the reduced size was instrumental in triggering and reproducing the recently fixed assertion race condition.

[jmid]

CI summary:

• Linux 5.1 debug aborted with an assertion failure in threadomain [ocaml5-issue] Assertion failure on threadomain under debug runtime #404
• Win bytecode timeouts (2 on 5.1, 1 on 5.2/trunk) during threadomain [ocaml5-issue] Windows failures on threadomain #203
• Win trunk crash on threadomain [ocaml5-issue] Windows failures on threadomain #203
• linux-s390x-5.1 failed to trigger a error in STM Weak test parallel in 2000 attempts STM Weak test parallel fail to trigger on s390x #406

Out of 63 workflows 6 failed, with 5 genuine failures and 1 false positive

[jmid]

CI summary for the merge to main: All green - 38 out of 38 runs succeeded!