CI: Fail if store paths are in the source

[grahamc]

Motivation

A customer noted that the tarball for Determinate Nix 3.15 cannot be fetched because the documentation contains "live" store path references in the documentation:

src/nix/ps.md
11:  nixbld11  3534394    0.8s  └───bash -e /nix/store/vj1c3wf9c11a0qs6p3ymfvrnsdgsdcbq-source-stdenv.sh /nix/store/shkw4qm9qcw5sc5n1k5jznc83ny02

doc/manual/source/release-notes-determinate/v3.14.0.md
13:_nixbld1  30167  0.2s  └───bash -e /nix/store/vj1c3wf9c11a0qs6p3ymfvrnsdgsdcbq-source-stdenv.sh /nix/store/shkw4qm9qcw5sc5n1k5jznc83ny02r39-default-builder.s

which causes fixed output fetchers to fail.

I propose we remediate this by replacing one character in these examples with an impossible character, like "o".

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Ideally, this PR fails CI immediately.

[github-actions]

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