Add an `is_musl` method to the `UbiBuilder` struct #353

Job	Run time
Alpine Linux	2m 8s
FreeBSD-x86_64 with rust stable	48s
Linux-x86_64 with rust stable	3m 7s
Linux-x86_64 (rustls-native-roots) with rust stable	2m 17s
Linux-x86_64 (native-tls) with rust stable	3m 6s
Linux-x86_64 (native-tls-vendored) with rust stable	4m 6s
Linux-aarch64 with rust stable	2m 30s
Linux-arm with rust stable	2m 28s
Linux-i686 with rust stable	2m 3s
Linux-powerpc with rust stable	1m 50s
Linux-powerpc64 with rust stable	1m 56s
Linux-powerpc64le with rust stable	1m 47s
Linux-riscv64 with rust stable	2m 9s
Linux-s390x with rust stable	1m 55s
NetBSD-x86_64 with rust stable	1m 58s
Windows-aarch64 with rust stable	4m 50s
Windows-i686 with rust stable	2m 51s
Windows-x86_64 with rust stable	6m 4s
macOS-x86_64 with rust stable	2m 35s
macOS-aarch64 with rust stable	1m 29s
Linux-x86_64 with rust beta	3m 18s
Linux-x86_64 with rust nightly	3m 9s
	58m 24s

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