vim_getenv: $VIMRUNTIME fallback: ../share/nvim/runtime (#6223)

Do this on all systems, so that portable builds work everywhere. This
allows us to ship archives with this folder structure:

    bin/nvim
    share/nvim/runtime

then ./bin/nvim works without the user needing to explicitly set VIMRUNTIME.

runtime/doc/starting.txt

@@ -657,15 +657,10 @@ is the order used to find the value of $VIMRUNTIME:
 3. If "$VIM/runtime" exists, it is used.
 4. The value of $VIM is used.  This is for backwards compatibility with older
    versions.
-5. When the 'helpfile' option is set and doesn't contain a '$', its value is
+5. If "../share/nvim/runtime" exists relative to |v:progpath|, it is used.
+6. When the 'helpfile' option is set and doesn't contain a '$', its value is
    used, with "doc/help.txt" removed from the end.
 
-For Unix, when there is a compiled-in default for $VIMRUNTIME (check the
-output of ":version"), steps 2, 3 and 4 are skipped, and the compiled-in
-default is used after step 5.  This means that the compiled-in default
-overrules the value of $VIM.  This is useful if $VIM is "/etc" and the runtime
-files are in "/usr/share/vim/vim54".
-
 Once Vim has done this once, it will set the $VIMRUNTIME environment variable.
 To change it later, use a ":let" command like this: >
 	:let $VIMRUNTIME = "/home/piet/vim/vim54"

src/nvim/os/env.c

@@ -615,8 +615,7 @@ char *vim_getenv(const char *name)
       vim_path = (char *)p_hf;
     }
 
-#ifdef WIN32
-    // Find runtime path relative to the nvim binary i.e. ../share/runtime
+    // Find runtime path relative to the nvim binary: ../share/nvim/runtime
     if (vim_path == NULL) {
       char exe_name[MAXPATHL];
       size_t exe_name_len = MAXPATHL;
@@ -633,7 +632,6 @@ char *vim_getenv(const char *name)
         }
       }
     }
-#endif
 
     if (vim_path != NULL) {
       // remove the file name