Refactoring of the libcoap-sys build script #62

Job	Run time
docs	2m 16s
lint	41s
test (libcoap-sys, openssl, msrv)	1m 59s
test (libcoap-sys, openssl, stable)	2m 1s
test (libcoap-sys, openssl, nightly)	2m 19s
test (libcoap-sys, gnutls, msrv)	1m 18s
test (libcoap-sys, gnutls, stable)	1m 46s
test (libcoap-sys, gnutls, nightly)	1m 42s
test (libcoap-sys, tinydtls, msrv)	1m 0s
test (libcoap-sys, tinydtls, stable)	1m 2s
test (libcoap-sys, tinydtls, nightly)	1m 1s
test (libcoap-sys, mbedtls, msrv)	1m 0s
test (libcoap-sys, mbedtls, stable)	1m 19s
test (libcoap-sys, mbedtls, nightly)	1m 6s
test (libcoap-rs, openssl, msrv)	2m 17s
test (libcoap-rs, openssl, stable)	2m 9s
test (libcoap-rs, openssl, nightly)	3m 0s
test (libcoap-rs, gnutls, msrv)	1m 0s
test (libcoap-rs, gnutls, stable)	57s
test (libcoap-rs, gnutls, nightly)	2m 50s
test (libcoap-rs, tinydtls, msrv)	1m 2s
test (libcoap-rs, tinydtls, stable)	2m 23s
test (libcoap-rs, tinydtls, nightly)	1m 9s
test (libcoap-rs, mbedtls, msrv)	1m 13s
test (libcoap-rs, mbedtls, stable)	1m 4s
test (libcoap-rs, mbedtls, nightly)	1m 12s
	40m 46s

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