Don't override default man search paths

By appending a colon to MANPATH NIX_MAN_DIR gets prepended to the
final MANPATH before default search paths.
This makes man still consider default search paths, but prefers
NIX_MAN_DIR (if it exists).

It still makes sense to point NIX_MAN_DIR to a correct location
by moving man pages build from nix-manual.man to nix-cli.man, but
this should fix most common use-cases where nix is installed globally.

Author: xokdvium

src/nix/man-pages.cc

@@ -13,7 +13,7 @@ std::filesystem::path getNixManDir()
 void showManPage(const std::string & name)
 {
     restoreProcessContext();
-    setEnv("MANPATH", getNixManDir().c_str());
+    setEnv("MANPATH", (getNixManDir().string() + ":").c_str());
     execlp("man", "man", name.c_str(), nullptr);
     if (errno == ENOENT) {
         // Not SysError because we don't want to suffix the errno, aka No such file or directory.