ci: move build jobs into a separate workflow (#539) #3

Job	Run time
Cross (sparcv9-sun-solaris)	36s
Cross (x86_64-unknown-illumos)	35s
Cross (x86_64-unknown-freebsd)	35s
ESP-IDF	22s
Cross (x86_64-unknown-netbsd)	37s
Apple Other	1m 40s
No Atomics	18s
RDRAND (x86_64-unknown-uefi)	29s
RDRAND (x86_64-unknown-l4re-uclibc)	26s
RNDR	22s
Wasm64	35s
Tier 3 (x86_64-unknown-hermit)	24s
Tier 2 (x86_64-unknown-fuchsia)	19s
Tier 3 (x86_64-wrs-vxworks)	28s
Tier 2 (x86_64-unknown-redox)	19s
Tier 3 (aarch64-kmc-solid_asp3)	27s
Tier 2 (x86_64-fortanix-unknown-sgx)	19s
Tier 3 (armv6k-nintendo-3ds)	25s
Tier 3 (armv7-sony-vita-newlibeabihf)	22s
Tier 3 (aarch64-unknown-nto-qnx710)	28s
Tier 3 (x86_64-unknown-openbsd, std)	29s
Tier 3 (x86_64-unknown-dragonfly, std)	31s
Tier 3 (x86_64-unknown-haiku, std)	30s
Tier 3 (i686-unknown-hurd-gnu, std)	31s
	12m 7s

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