Embedded builds to use $workspace_root/etc again if run from build dir

Commit f05ad46980 (config_paths: remove vestiges of installable
builds, 2025-09-06) removed a bunch of code paths for embed-data
builds, since those builds can do without most config paths.

However they still want the sysconfig path.  That commit made
embedded builds use "/etc/fish" unconditionally.  Previously they
used "$workspace_root/etc".  This is important when running tests,
which should not read /etc/fish.

tests/checks/invocation.fish tests this implicitly: if /etc/fish does
not exist, then

	fish --profile-startup /dev/stdout

will not contain "builtin source".

Let's restore historical behavior.  This might be annoying for users
who "install" with "ln -s target/debug/fish ~/bin/", but that hasn't
ever been recommended, and the historical behavior was in effect
until 4.1.0.

Fixes #11900

src/env/config_paths.rs

@@ -1,7 +1,7 @@
 use crate::common::wcs2string;
+use crate::common::BUILD_DIR;
 use crate::wchar::prelude::*;
 use crate::{common::get_executable_path, FLOG, FLOGF};
-#[cfg(not(feature = "embed-data"))]
 use fish_build_helper::workspace_root;
 use std::ffi::OsString;
 use std::os::unix::ffi::OsStringExt;
@@ -63,7 +63,14 @@ fn static_paths() -> Self {
     fn from_exec_path(exec_path: PathBuf) -> Self {
         FLOG!(config, "embed-data feature is active, ignoring data paths");
         Self {
-            sysconf: PathBuf::from(SYSCONF_DIR).join("fish"),
+            sysconf: if exec_path
+                .canonicalize()
+                .is_ok_and(|exec_path| exec_path.starts_with(BUILD_DIR))
+            {
+                workspace_root().join("etc")
+            } else {
+                PathBuf::from(SYSCONF_DIR).join("fish")
+            },
             bin: exec_path.parent().map(|x| x.to_path_buf()),
         }
     }
@@ -72,8 +79,6 @@ fn from_exec_path(exec_path: PathBuf) -> Self {
     fn from_exec_path(unresolved_exec_path: PathBuf) -> Self {
         use std::path::Path;
 
-        use crate::common::BUILD_DIR;
-
         let invalid_exec_path = |exec_path: &Path| {
             FLOG!(
                 config,

tests/checks/config-paths-standalone.fish

@@ -0,0 +1,12 @@
+# RUN: %fish -d config | grep -v ^Debug.enabled
+# REQUIRES: %fish -c 'status build-info' | grep '^Features:.*embed-data'
+
+# CHECKERR: config: executable path: {{.*}}/fish
+# CHECKERR: config: embed-data feature is active, ignoring data paths
+
+# NOTE: When our executable is located outside the workspace, this is "/etc".
+# CHECKERR: config: paths.sysconf: {{.+}}/etc
+
+# CHECKERR: config: paths.bin: {{.*}}
+# CHECKERR: config: sourcing /home/johannes/git/fish-shell/etc/config.fish
+# CHECKERR: config: not sourcing {{.*}}/xdg_config_home/fish/config.fish (not readable or does not exist)