g.extension: fix addon SOURCE_URL env var value for official GitHub addons

[tmszi]

Describe the bug
Installed extension HTML manual page has wrong source code and history GitHub URL link.

Wrong is SOURCE_URL env var value, which is for every official addon https://github.com/OSGeo/grass-addons (correct URL for e.g. db.join addon should be https://github.com/OSGeo/grass-addons/tree/grass8/src/db/db.join):

grass/scripts/g.extension/g.extension.py

Line 2091 in c0addd9

"SOURCE_URL=%s" % url,

which is used inside mkhtml.py file (line 802)

grass/utils/mkhtml.py

Lines 791 to 804 in c0addd9

	if os.getenv("SOURCE_URL", ""):
	addon_path = get_addon_path()
	if addon_path:
	# Addon is installed from the local dir
	if os.path.exists(os.getenv("SOURCE_URL")):
	url_source = urlparse.urljoin(
	addons_url,
	addon_path,
	)
	else:
	url_source = urlparse.urljoin(
	os.environ["SOURCE_URL"].split("src")[0],
	addon_path,
	)

To Reproduce
Steps to reproduce the behavior:

1. Try install some extension e.g. g.extension db.join
2. Check db.join HTML manual page with $EDITOR $GRASS_ADDON_BASE/docs/html/db.join.html
3. See SOURCE code section db.join source code and history link

<h2>SOURCE CODE</h2>
<p>
  Available at:
  <a href="https://github.com/OSGeo/src/db/db.join">db.join source code</a>
  (<a href="https://github.com/OSGeo/src/db/db.join">history</a>)
</p>

Check if these URLs exists:

test@test:~$ URL=https://github.com/OSGeo/src/db/db.join; if curl --output /dev/null --silent --head --fail "$URL"; then echo "URL exists: $URL"; else echo "URL does not exist: $URL"; fi
URL does not exist: https://github.com/OSGeo/src/db/db.join

Expected behavior
Installed extension HTML manual page should have correct source code and history GitHub URL link.

e.g. db.join addon

<h2>SOURCE CODE</h2>
<p>
  Available at:
  <a href="https://github.com/OSGeo/grass-addons/tree/grass8/src/db/db.join">db.join source code</a>
  (<a href="https://github.com/OSGeo/grass-addons/commits/grass8/src/db/db.join">history</a>)
</p>

Check if these URLs exists:

test@test:~$ URL=https://github.com/OSGeo/grass-addons/tree/grass8/src/db/db.join; if curl --output /dev/null --silent --head --fail "$URL"; then echo "URL exists: $URL"; else echo "URL does not exist: $URL"; fi
URL exists: https://github.com/OSGeo/grass-addons/tree/grass8/src/db/db.join

test@test:~$ URL=https://github.com/OSGeo/grass-addons/commits/grass8/src/db/db.join; if curl --output /dev/null --silent --head --fail "$URL"; then echo "URL exists: $URL"; else echo "URL does not exist: $URL"; fi
URL exists: https://github.com/OSGeo/grass-addons/commits/grass8/src/db/db.join

System description (please complete the following information):

• Operating System: UNIX like
• GRASS GIS version 8.3.dev

Additional context
To be backported with #2895.