Pin our tracing-core dep due to binary size concerns #1290

Job	Run time
build mdbook guide	1m 31s
build artifacts [x64-linux]	5m 52s
build and check docs [x64-windows]	9m 22s
build openhcl [aarch64-linux]	5m 24s
build openhcl [x64-linux]	7m 40s
clippy [windows], unit tests [x64-windows]	12m 45s
clippy [linux-musl, misc nostd], unit tests [x64-linux-musl]	7m 8s
clippy [linux, macos], unit tests [x64-linux]	8m 9s
unit tests [aarch64-windows]	6m 46s
build and check docs [x64-linux]	2m 44s
test flowey local backend	3m 5s
xtask fmt (linux)	2m 23s
build artifacts (not for VMM tests) [aarch64-windows]	5m 40s
build artifacts (for VMM tests) [aarch64-windows]	8m 55s
build artifacts (not for VMM tests) [x64-windows]	5m 8s
build artifacts (for VMM tests) [x64-windows]	10m 26s
build artifacts [aarch64-linux]	5m 24s
xtask fmt (windows)	6m 49s
run vmm-tests [aarch64-windows]	5m 44s
run vmm-tests [x64-windows-intel]	8m 56s
run vmm-tests [x64-linux]	3m 38s
run vmm-tests [x64-windows-amd]	11m 17s
openvmm checkin gates	8s
	2h 24m 54s

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