Add bootstrap test on Alpine Linux #357

Job	Run time
Alpine Linux	2m 5s
FreeBSD-x86_64 with rust stable	56s
Linux-x86_64 with rust stable	3m 6s
Linux-x86_64 (rustls-native-roots) with rust stable	3m 6s
Linux-x86_64 (native-tls) with rust stable	3m 2s
Linux-x86_64 (native-tls-vendored) with rust stable	3m 7s
Linux-aarch64 with rust stable	2m 35s
Linux-arm with rust stable	2m 31s
Linux-i686 with rust stable	2m 5s
Linux-powerpc with rust stable	45s
Linux-powerpc64 with rust stable	1m 32s
Linux-powerpc64le with rust stable	2m 0s
Linux-riscv64 with rust stable	2m 22s
Linux-s390x with rust stable	2m 6s
NetBSD-x86_64 with rust stable	2m 14s
Windows-aarch64 with rust stable	2m 49s
Windows-i686 with rust stable	1m 48s
Windows-x86_64 with rust stable	1m 56s
macOS-x86_64 with rust stable	2m 11s
macOS-aarch64 with rust stable	1m 1s
Linux-x86_64 with rust beta	3m 6s
Linux-x86_64 with rust nightly	3m 4s
	49m 27s

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