Setup Fastly surrogate key for purging manifests.txt

[Mark-Simulacrum]

We've noticed that the manifests.txt cache on Fastly is sometimes outdated after a new nightly is published. This is purge by promote-release here (https://github.com/rust-lang/promote-release/blob/44a5f66b1e0b9d780a8737abcbe41587e952dbb0/src/fastly.rs#L19, https://github.com/rust-lang/promote-release/blob/44a5f66b1e0b9d780a8737abcbe41587e952dbb0/src/main.rs#L611). That code generally speaking works for CloudFront because CloudFront supports path-based invalidation. Fastly requires 'tags' to be set on URLs at request time, and then those are invalidated later: https://www.fastly.com/documentation/reference/api/purging/#purge-tag

We will need to extend

simpleinfra/terragrunt/modules/release-distribution/fastly-static.tf

Lines 82 to 90 in 5fe9fb7

	snippet {
	name = "set cache key for dist"
	type = "fetch"
	content = <<-VCL
	if (req.url ~ "^\/dist\/") {
	set beresp.http.Surrogate-Key = "dist";
	}
	VCL
	}

to add a tag (surrogate key) to the manifests.txt object and wire up purging for it. That tag should be manifeststxt per https://github.com/rust-lang/promote-release/blob/44a5f66b1e0b9d780a8737abcbe41587e952dbb0/src/fastly.rs#L48.

cc https://rust-lang.zulipchat.com/#narrow/channel/241545-t-release/topic/manifests.2Etxt.20CDN.20outdated/with/560117719

[Turbo87]

FWIW fastly also allows purging of single URLs without surrogate keys: https://www.fastly.com/documentation/reference/api/purging/#purge-single-url

that's what we're currently using for crates.io

[Mark-Simulacrum]

I'd be happy to accept a PR to promote-release to switch invalidations of manifests.txt to that form, that does seem easier.