Fix truncated package metadata in additional packages preview (#3580)

[Softer]

Fixes #3580.

Summary

pacman -S --info wraps long fields like Description across multiple lines when the pty width is small (default 80 cols). archinstall's package info parser pre-stripped each output line before joining continuations, which discarded the leading whitespace it relied on to detect continuation lines, so the wrapped portions were silently dropped.

Example before fix, for sqlite-doc:

Description : most of the static HTML files that comprise this website,

After fix:

Description : most of the static HTML files that comprise this website,
including all of the SQL Syntax and the C/C++ interface specs
and other miscellaneous documentation

What changed

• Pacman.run() gained an optional environment_vars parameter that is forwarded to SysCommand.
• The two pacman -S --info call sites in archinstall/lib/packages/packages.py now pass {'COLUMNS': '500'}, forcing pacman to emit single-line field values.
• AvailablePackage.info() now word-wraps each value to roughly half the terminal width using textwrap.fill, so the preview pane does not get a horizontal scrollbar on long fields. The continuation lines are aligned to the value column to match pacman's own visual style.

Scope is limited to -S --info calls; sync/install paths are unaffected.

[h8d13]

I don't think you need the env var plumbing if you fix parser call sites