install_go_toolchain downloaded the tarball to a fixed /tmp name and
unpacked it into /usr/local as root, with no digest check. Another
local user could plant that file first, and nothing would notice a
tampered download.
The tarball is now staged in a mktemp -d directory that the exit
cleanup removes, and its sha256 must match before the old toolchain is
touched, so a refusal leaves the host as it was. The default 1.26.4
carries pinned amd64 and arm64 digests next to its version; they are
the ones https://go.dev/dl/?mode=json&include=all publishes. Any other
FELIS_GO_VERSION has to bring its own FELIS_GO_SHA256, documented in
the header, because no pin can cover a version chosen at run time.
Where and which version gets installed is unchanged.
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