fix(popup): make sure border buf is modifiable

[folke]

When creating a popup on a nonmodifiable buffer, creating the border fails, probably becasue it inherits the nonmodifable option.

Fixes https://github.com/folke/noice.nvim/issues/567

[codecov[bot]]

Codecov Report

Merging #296 (7fb8b19) into main (0f913a3) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #296   +/-   ##
=======================================
  Coverage   99.25%   99.25%           
=======================================
  Files          20       20           
  Lines        2694     2695    +1     
=======================================
+ Hits         2674     2675    +1     
  Misses         20       20           

Files Changed	Coverage Δ
lua/nui/popup/border.lua	100.00% <100.00%> (ø)

[folke]

Neither did I tbh. inheritance of options in Vim is weird sometimes.