Create NerdFontsSymbolsOnly with proportional widths

[why]
The commit
  59c45ba  Remove negative bearings on 2048-em glyphs
has been introduced to fix some problems with the symbols only font, at
least from the commit message.

That font is intended to be used in font-fallback situations, and so we
do not know the advance width of the current font anyhow. It does not
make sense to enforce an advance width with these.

[how]
Create the NerdFontsSymbolsOnly as if 59c45ba would be still in
effect, i.e. with variable advance width.

[note]
There have been a lot discussions about the reverted commit, some can be
found here:
* #900
* #764
* #731

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

src/unpatched-fonts/NerdFontsSymbolsOnly/config.cfg

@@ -1 +1 @@
-config_patch_flags="--ext ttf"
+config_patch_flags="--ext ttf --variable-width-glyphs"