Use `OnceLock` in `JobTokenServer::new` #1417

Job	Run time
Test CUDA support	23s
Test (stable)	43s
Clippy	9s
Test (beta)	43s
Test build-std (aarch64-tvos)	3m 30s
MSRV (ubuntu-latest)	29s
Test (nightly)	37s
Test build-std (aarch64-tvos-sim)	2m 34s
MSRV (windows-latest)	46s
Rustfmt	12s
Test (linux32)	1m 23s
Test build-std (x86_64-tvos)	3m 7s
Test (macos)	3m 32s
Test (aarch64-macos)	51s
Test (aarch64-ios)	1m 17s
Test (win32)	1m 51s
Test (win64)	2m 45s
Test (mingw32)	1m 48s
Test (mingw64)	1m 53s
Test (windows-2019)	2m 31s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	1m 59s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	2m 28s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	1m 56s
	37m 27s

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