Various CI pipeline improvements #202

Job	Run time
rustfmt (libcoap-sys)	6s
test (libcoap-sys, openssl, msrv)	2m 7s
rustfmt (libcoap-rs)	7s
test (libcoap-sys, openssl, stable)	2m 0s
lint (libcoap-sys)	1m 48s
test (libcoap-sys, openssl, nightly)	2m 29s
lint (libcoap-rs)	1m 57s
test (libcoap-sys, gnutls, msrv)	57s
test (libcoap-sys, gnutls, stable)	52s
test (libcoap-sys, gnutls, nightly)	1m 16s
test (libcoap-sys, tinydtls, msrv)	54s
test (libcoap-sys, tinydtls, stable)	56s
test (libcoap-sys, tinydtls, nightly)	1m 16s
test (libcoap-sys, mbedtls, msrv)	58s
test (libcoap-sys, mbedtls, stable)	56s
test (libcoap-sys, mbedtls, nightly)	1m 19s
test (libcoap-rs, openssl, msrv)	2m 5s
test (libcoap-rs, openssl, stable)	1m 59s
test (libcoap-rs, openssl, nightly)	2m 8s
test (libcoap-rs, gnutls, msrv)	54s
test (libcoap-rs, gnutls, stable)	1m 30s
test (libcoap-rs, gnutls, nightly)	1m 2s
test (libcoap-rs, tinydtls, msrv)	1m 0s
test (libcoap-rs, tinydtls, stable)	59s
test (libcoap-rs, tinydtls, nightly)	1m 18s
test (libcoap-rs, mbedtls, msrv)	1m 2s
test (libcoap-rs, mbedtls, stable)	58s
test (libcoap-rs, mbedtls, nightly)	1m 17s
coverage	48s
pr-comment	5s
	37m 3s

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