Lock rb-sys to 0.9.106

[gjtorikian]

For an inexplicable reason, the cross-compile action passed in a PR but failed when that PR was merged into main.

Digging into it, one thing caught my eye:

Installing [email protected]
Successfully installed rb_sys-0.9.87
1 gem installed
...
::info::🐳 Building for Ruby requested versions: ["3.1.0", "3.2.0", "3.3.0", "3.4.0"]
::info::🐳 Downloading container "rbsys/x86_64-linux:0.9.105", this might take awhile...

The rb_sys gem and rbsys docker image should always be matching. 0.9.106 contains a necessary fix for Rubies > 3.2, so it might as well point to that.