Fix jq errors in prerelease script

[bycEEE]

The current https://raw.githubusercontent.com/SteamDeckHomebrew/decky-loader/main/dist/install_prerelease.sh script doesn't work and errors out when trying to execute.

(deck@breezydeck ~)$ ./install_prerelease.sh
Installing Steam Deck Plugin Loader pre-release...
./install_prerelease.sh: line 16: [: too many arguments
curl: no URL specified!
curl: try 'curl --help' for more information

This is a quick fix to pass in the variables to jq correctly to rectify this issue.

[bycEEE]

Second commit has the following changes:

• Add back the removed $RELEASE variable in case you want to use it later for something else.
• Assign multiple variables at once instead of calling jq again for each one.
• Follow the variable expansion paradigm already set (eg: $RELEASE -> ${RELEASE}.
• Add version number in installation output.

  Installing Steam Deck Plugin Loader pre-release...
  Installing version v2.0.4-pre...
    % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                   Dload  Upload   Total   Spent    Left  Speed
    0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  100 18.1M  100 18.1M    0     0  12.0M      0  0:00:01  0:00:01 --:--:-- 23.6M
  Created symlink /etc/systemd/system/multi-user.target.wants/plugin_loader.service → /etc/systemd/system/plugin_loader.service.