Various CI pipeline improvements #42

Job	Run time
test (libcoap-sys, openssl, msrv)	1m 58s
lint	1m 1s
test (libcoap-sys, openssl, stable)	2m 10s
test (libcoap-sys, openssl, nightly)	2m 1s
test (libcoap-sys, gnutls, msrv)	53s
test (libcoap-sys, gnutls, stable)	1m 1s
test (libcoap-sys, gnutls, nightly)	1m 6s
test (libcoap-sys, tinydtls, msrv)	1m 1s
test (libcoap-sys, tinydtls, stable)	51s
test (libcoap-sys, tinydtls, nightly)	59s
test (libcoap-sys, mbedtls, msrv)	1m 7s
test (libcoap-sys, mbedtls, stable)	58s
test (libcoap-sys, mbedtls, nightly)	1m 11s
test (libcoap-rs, openssl, msrv)	2m 6s
test (libcoap-rs, openssl, stable)	2m 1s
test (libcoap-rs, openssl, nightly)	2m 19s
test (libcoap-rs, gnutls, msrv)	1m 20s
test (libcoap-rs, gnutls, stable)	53s
test (libcoap-rs, gnutls, nightly)	1m 12s
test (libcoap-rs, tinydtls, msrv)	59s
test (libcoap-rs, tinydtls, stable)	1m 22s
test (libcoap-rs, tinydtls, nightly)	56s
test (libcoap-rs, mbedtls, msrv)	1m 8s
test (libcoap-rs, mbedtls, stable)	1m 4s
test (libcoap-rs, mbedtls, nightly)	1m 11s
	32m 48s

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