Add install.sh one-line bootstrap, published next to the binaries

Every manifest repeated the same four lines to get the binary: mkdir,
curl, chmod, and a PATH export into ~/.buildenv. install.sh does all of
it, picks the asset for the host platform, and verifies it against the
published checksums.txt — which the hand-rolled version never did.

On builds.sr.ht ~/.buildenv is sourced before each task, so the PATH
export lands from the next task onwards; keep install and `cacher init`
in separate tasks.

The checksums.txt glob stays cacher-*: install.sh must not appear in the
manifest it verifies against.