dune subst silently ignores %%ID%% string when outside a git-controlled directory

[btjorge]

Note this impact file-pinned opam packages in particular

Reproduction

$ mkdir test

$ cat > dune-project << EOF
(lang dune 3.16)
(name test)
EOF

$ echo "%%NAME%%" > README.md

Expected Behavior

$ dune subst
$ cat README.md
test

Actual Behavior

$ dune subst
$ cat README.md
%%NAME%%

Specifications

• dune 3.16.0
• ocaml 5.2
• debian 12