fix: recognise Discord's bootstrap launcher

Discord no longer ships its application. /usr/bin/discord is a few lines
of shell that run updater_bootstrap, which downloads Electron into the
user's config directory on first start and execs it from there. So there
is no binary next to the launcher to find markers around, and the path it
hands over to is built out of $HOME and the version it just downloaded,
which the script reader cannot resolve. Discord came out as "not
Chromium" and was dropped from the list entirely.

The name of the bootstrap it ships is the one thing left that says
Discord, so that is the hint now.

src/lib/detect.sh

@@ -74,7 +74,14 @@ MCA_SNAP_DIRS=(/snap /var/lib/snapd/snap)
 # CHROMIUM_FLAGS and CHROME_WRAPPER earn their place: they are the variables
 # the Debian, Fedora and openSUSE Chromium wrappers and Google's own Chrome
 # wrapper build their command line out of, and nothing else sets them.
-MCA_SCRIPT_HINTS='ELECTRON_|app\.asar|chrome-sandbox|libcef|enable-blink-features|ozone-platform-hint|CHROMIUM_FLAGS|CHROME_WRAPPER|CHROME_VERSION_EXTRA'
+#
+# updater_bootstrap is Discord's, and it is the only thing left that names it.
+# Discord no longer ships its application: /usr/bin/discord is a few lines of
+# shell that run that bootstrap, which downloads Electron into the user's
+# config directory on first start and execs it from there. So there is nothing
+# next to the launcher to find, and the path it hands over to is built out of
+# $HOME and the version it just downloaded.
+MCA_SCRIPT_HINTS='ELECTRON_|app\.asar|chrome-sandbox|libcef|enable-blink-features|ozone-platform-hint|CHROMIUM_FLAGS|CHROME_WRAPPER|CHROME_VERSION_EXTRA|updater_bootstrap'
 
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 # Desktop entries