Refactoring of the libcoap-sys build script #60

Job	Run time
lint	36s
docs	2m 8s
test (libcoap-sys, openssl, msrv)	2m 9s
test (libcoap-sys, openssl, stable)	2m 7s
test (libcoap-sys, openssl, nightly)	2m 0s
test (libcoap-sys, gnutls, msrv)	1m 7s
test (libcoap-sys, gnutls, stable)	54s
test (libcoap-sys, gnutls, nightly)	58s
test (libcoap-sys, tinydtls, msrv)	59s
test (libcoap-sys, tinydtls, stable)	57s
test (libcoap-sys, tinydtls, nightly)	59s
test (libcoap-sys, mbedtls, msrv)	58s
test (libcoap-sys, mbedtls, stable)	59s
test (libcoap-sys, mbedtls, nightly)	1m 2s
test (libcoap-rs, openssl, msrv)	2m 14s
test (libcoap-rs, openssl, stable)	2m 6s
test (libcoap-rs, openssl, nightly)	2m 20s
test (libcoap-rs, gnutls, msrv)	56s
test (libcoap-rs, gnutls, stable)	1m 2s
test (libcoap-rs, gnutls, nightly)	1m 4s
test (libcoap-rs, tinydtls, msrv)	1m 3s
test (libcoap-rs, tinydtls, stable)	1m 5s
test (libcoap-rs, tinydtls, nightly)	1m 9s
test (libcoap-rs, mbedtls, msrv)	1m 3s
test (libcoap-rs, mbedtls, stable)	1m 3s
test (libcoap-rs, mbedtls, nightly)	1m 9s
	34m 7s

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