Do not embed man pages in CMake builds

Commit 0709e4be8b (Use standalone code paths by default, 2025-10-26)
mainly wanted to embed internal functions to unbreak upgrade scenarios.

Embedding man pages in CMake has small-ish disadvantages:
1. extra space usage
2. less discoverability
3. a "cyclic" dependency:
   1. "sphinx-docs" depends on "fish_indent"
   2. "fish_indent" via "crates/build-man-pages" depends on "doc_src/".
   So every "touch doc_src/foo.rst && ninja -Cbuild sphinx-docs"
   re-builds fish, just to re-run sphinx-build.

The significant one is number 3.  It can be worked around by running
sphinx-build with stale "fish_indent" but I don't think we want to
do that.

Let's backtrack a little by stopping embedding man pages in CMake
builds; use the on-disk man pages (which we still install).

The remaining "regression" from 0709e4be8b is that "ninja -Cbuild
fish" needs to rebuild whenever anything in "share/" changes.  I don't
know if that's also annoying?

Since man pages for "build/fish" are not in share/, expose the exact
path as $__fish_man_dir.

tests/checks/__fish_tried_to_embed_manpages.fish

@@ -0,0 +1,13 @@
+# RUN: %fish %s
+
+set -l value (string join , \
+    embed-manpages="$(__fish_tried_to_embed_manpages && echo true || echo false)" \
+    "build-system=$(status build-info | string replace -rf 'Build system: (.*)' '$1')"
+)
+switch $value
+    case embed-manpages=true,build-system=Cargo
+    case embed-manpages=false,build-system=CMake
+    case '*'
+        echo Expected embedded man pages to be detected as true iff using Cargo
+        echo got: $value
+end

tests/checks/config-paths.fish

@@ -11,6 +11,7 @@
 
 # NOTE: When our executable is located outside the build directory, these are different.
 # CHECKERR: config: paths.data: {{.*}}/share
+# CHECKERR: config: paths.man: {{.*/user_doc/man|\|not found\|}}
 # CHECKERR: config: paths.doc: {{.*/user_doc/html|\|not found\|}}
 
 # CHECKERR: config: sourcing {{.+}}/etc/config.fish

tests/checks/man.fish

@@ -1,12 +1,12 @@
 # RUN: %fish %s
 
-# Override the test-override again.
-__fish_data_with_file functions/__fish_print_help.fish source
-
 # REQUIRES: command -v sphinx-build
 # REQUIRES: command -v man
 # REQUIRES: test "$FISH_BUILD_DOCS" != "0"
 
+# Override the test-override again.
+__fish_data_with_file functions/__fish_print_help.fish source
+
 set -l deroff col -b -p -x
 
 set -lx MANWIDTH 80