"--portable": more consistent behaviour

The old and new behaviour is as follows:
1. "pyinstaller" case: portable `.exe`, other `.exe`s with `--portable`, and `.dmg` with `--portable`
    - uses `$PWD`
    - note that when you double-click the portable `.exe` on Windows, `$PWD` is set to the parent folder, i.e. the datadir gets put next to the `.exe`
2. appimage `--portable`
    - was broken (see #5551)
    - (CHANGED NOW to) uses `$PWD`
3. git clone
    - next to `run_electrum`
4. unpacking `tar.gz` and running locally from it
    - next to `run_electrum`
5. `pip install *.tar.gz`, and calling "electrum --portable" from terminal
    - used python's user script directory
        - `~/.local/bin/electrum_data`
        - `$VIRTUAL_ENV/bin/electrum_data`
    - (CHANGED NOW to) uses `$PWD`

That is, we now almost always put the datadir in `$PWD`,
except for the local source case, where we put it next to `run_electrum`.

The "appimage" case (2) is now fixed.

The only breaking change is re case 5 which previously behaved completely
unintuitively and most likely not in a useful way.

closes #7732
fixes #5551

run_electrum

@@ -338,10 +338,17 @@ def main():
         config_options['portable'] = True
 
     if config_options.get('portable'):
-        if is_pyinstaller:
-            datadir = os.path.join(os.path.realpath(cwd), 'electrum_data')
-        else:
+        if is_local:
+            # running from git clone or local source: put datadir next to main script
             datadir = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'electrum_data')
+        else:
+            # Running a binary or installed source. The most generic but still reasonable thing
+            # is to use the current working directory. (see #7732)
+            # note: The main script is often unpacked to a temporary directory from a bundled executable,
+            #       and we don't want to put the datadir inside a temp dir.
+            # note: Re the portable .exe on Windows, when the user double-clicks it, CWD gets set
+            #       to the parent dir, i.e. we will put the datadir next to the exe.
+            datadir = os.path.join(os.path.realpath(cwd), 'electrum_data')
         config_options['electrum_path'] = datadir
 
     if not config_options.get('verbosity'):