Fix filename generation for many platforms in bootstrap scripts #350

Job	Run time
Alpine Linux	1m 58s
FreeBSD-x86_64 with rust stable	1m 4s
Linux-x86_64 with rust stable	3m 8s
Linux-x86_64 (rustls-native-roots) with rust stable	2m 13s
Linux-x86_64 (native-tls) with rust stable	1m 15s
Linux-x86_64 (native-tls-vendored) with rust stable	4m 16s
Linux-aarch64 with rust stable	2m 57s
Linux-arm with rust stable	2m 42s
Linux-i686 with rust stable	2m 6s
Linux-powerpc with rust stable	1m 47s
Linux-powerpc64 with rust stable	2m 0s
Linux-powerpc64le with rust stable	1m 50s
Linux-riscv64 with rust stable	2m 19s
Linux-s390x with rust stable	1m 50s
NetBSD-x86_64 with rust stable	2m 0s
Windows-aarch64 with rust stable	4m 9s
Windows-i686 with rust stable	3m 22s
Windows-x86_64 with rust stable	6m 45s
macOS-x86_64 with rust stable	2m 3s
macOS-aarch64 with rust stable	1m 5s
Linux-x86_64 with rust beta	3m 8s
Linux-x86_64 with rust nightly	3m 0s
	56m 57s

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