test refc in CI in Nim 2.0 and later

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[NimScript] exec: nim cpp  --styleCheck:usages --styleCheck:error --verbosity:0 --hints:off --skipParentCfg --skipUserCfg --outdir:build --nimcache:build/nimcache -f  -d:asyncBackend=none --threads:on -d:useMalloc -r tests/all_tests
D:\a\nim-faststreams\nim-faststreams\faststreams\textio.nim(4, 9) Warning: use `std/formatfloat`; formatfloat is deprecated [Deprecated]
D:\a\nim-faststreams\nim-faststreams\faststreams\textio.nim(4, 9) Warning: imported and not used: 'formatfloat' [UnusedImport]
input stream  (0.00s)
memoryInput empty inputs ... (0.00s)
unsafeMemoryInput empty inputs ... (0.00s)
fileInput empty inputs ... (0.00s)
memFileInput empty inputs ... (0.00s)
count lines ... (0.02s)
mixed read types ... (0.00s)
misc ... (0.00s)
input stream ... (0.00s)
output stream ........ (0.16s)
randomized tests ..... (7.11s)
No stack traceback available
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
Error: execution of an external program failed: 'D:\a\nim-faststreams\nim-faststreams\build\all_tests.exe'
stack trace: (most recent call last)
D:\a\_temp\msys64\tmp\nimblecache-3274857584\nimscriptapi_3388400695.nim(210, 16)
D:\a\nim-faststreams\nim-faststreams\faststreams.nimble(37, 11) testTask
D:\a\nim-faststreams\nim-faststreams\faststreams.nimble(28, 3) run
D:\a\nim-faststreams\nim-faststreams\faststreams.nimble(25, 3) build
D:\a\nim-faststreams\nim-faststreams\nim\lib\system\nimscript.nim(265, 7) exec
D:\a\nim-faststreams\nim-faststreams\nim\lib\system\nimscript.nim(265, 7) Error: unhandled exception: FAILED: nim cpp  --styleCheck:usages --styleCheck:error --verbosity:0 --hints:off --skipParentCfg --skipUserCfg --outdir:build --nimcache:build/nimcache -f  -d:asyncBackend=none --threads:on -d:useMalloc -r tests/all_tests [OSError]

Well, now macOS 2.0 builds fine, but Windows (non-refc, so unaffected by this PR, so will merge this PR) C++ backend now fails under ORC in 2.0.