libstore: escape store paths when unparsing derivations

[awsmadi]

printUnquotedString escapes nothing, so store paths written through it land in the
.drv verbatim. Harmless on Unix, where store paths contain no character
printString would escape. On Windows they contain backslashes, and the reader's
escape table then mis-decodes them: \n in ...\nix\store becomes a literal
newline and the path fails to round-trip.

The artefact indicts itself. One .drv carried the same store path twice, unescaped
in the outputs field and escaped in the env field:

Derive([("out","C:\ProgramData\nix\store/bk8vf...-winhello",...
   ...  [("out","C:\\ProgramData\\nix\\store/bk8vf...-winhello"),

Reading it back gave path 'C:ProgramData<newline>ixstore/...' is not in the Nix store.

This switches the four store-path writes in unparse to printString.

Nothing can change on Unix. printString is a superset of printUnquotedString:
for input containing none of ", \, \n, \r, \t the two produce identical
bytes, and store paths on Unix contain none of them. So no .drv, derivation hash
or cache entry moves. Checked rather than assumed: nix-store-tests is unmoved at
764 passed / 12 failed across both arms with the same twelve names, and a
deliberately different store hash does move the output, so the comparison could
have detected a change.

This does not make building work on Windows. Realising a derivation stops earlier
for want of a builder.

[Ericson2314]

Let's do this after #16284