.github, Dockerfile, runner: explicitly disable threads

[ee7]

Nim 2.0 will enable threads by default. Let's handle the enabling of threads independently from the Nim bump.

To enable threads, we'd need to:

• add a test for threads
• and either:
  • add --tlsEmulation:on to allow compiling with tcc, and probably update tcc too
  • stop using tcc

Refs: #93

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See failing CI log from the Nim 2.0 bump PR.