appveyor: Reformat an absurdly long line in .appveyor.yml

Using YAML line-folding doesn't seem possible, as setup doesn't tolerate
spaces between package names, so instead drop some unneeded double quotes,
double quote the whole thing and escape newlines

.appveyor.yml

@@ -15,7 +15,65 @@ install:
 - if "%BUILD%"=="i686-pc-cygwin" set SETUP=setup-x86.exe && set CYGWIN_ROOT=C:\cygwin
 - curl -fsSL https://gist.githubusercontent.com/jon-turney/0338af595313f598bfab15a0ac0df847/raw/bd0eeca6be899e7846aa988fbcf15e4e12f5f842/zp_libtool_cleanlafiles.sh -o %CYGWIN_ROOT%\etc\postinstall\zp_libtool_cleanlafiles.sh
 - echo Updating Cygwin and installing build prerequisites
-- '%CYGWIN_ROOT%\%SETUP% -qnNdO -R "%CYGWIN_ROOT%" -s "%CYGWIN_MIRROR%" -l "%CACHE%" -g -P "meson,binutils,bison,bzip2,diffutils,fileutils,findutils,flex,gawk,gcc-core,make,patch,pkg-config,python3,sed,tar,xorgproto,windowsdriproto,libdmx-devel,libfontenc-devel,libfreetype-devel,libGL-devel,libpixman1-devel,libX11-devel,libXRes-devel,libXau-devel,libXaw-devel,libXdmcp-devel,libXext-devel,libXfont2-devel,libXi-devel,libXinerama-devel,libXmu-devel,libXpm-devel,libXrender-devel,libXtst-devel,libxcb-ewmh-devel,libxcb-icccm-devel,libxcb-image-devel,libxcb-keysyms-devel,libxcb-randr-devel,libxcb-render-devel,libxcb-render-util-devel,libxcb-shape-devel,libxcb-util-devel,libxcb-xkb-devel,libxkbfile-devel,zlib,font-util,khronos-opengl-registry,python3-lxml,xorg-util-macros,xtrans,xkbcomp,xkeyboard-config,libnettle-devel,libepoxy-devel,libtirpc-devel"'
+- "%CYGWIN_ROOT%\\%SETUP% -qnNdO -R %CYGWIN_ROOT% -s %CYGWIN_MIRROR% -l %CACHE% -g -P \
+meson,\
+binutils,\
+bison,\
+bzip2,\
+diffutils,\
+fileutils,\
+findutils,\
+flex,\
+gawk,\
+gcc-core,\
+make,\
+patch,\
+pkg-config,\
+python3,\
+sed,\
+tar,\
+xorgproto,\
+windowsdriproto,\
+libdmx-devel,\
+libfontenc-devel,\
+libfreetype-devel,\
+libGL-devel,\
+libpixman1-devel,\
+libX11-devel,\
+libXRes-devel,\
+libXau-devel,\
+libXaw-devel,\
+libXdmcp-devel,\
+libXext-devel,\
+libXfont2-devel,\
+libXi-devel,\
+libXinerama-devel,\
+libXmu-devel,\
+libXpm-devel,\
+libXrender-devel,\
+libXtst-devel,\
+libxcb-ewmh-devel,\
+libxcb-icccm-devel,\
+libxcb-image-devel,\
+libxcb-keysyms-devel,\
+libxcb-randr-devel,\
+libxcb-render-devel,\
+libxcb-render-util-devel,\
+libxcb-shape-devel,\
+libxcb-util-devel,\
+libxcb-xkb-devel,\
+libxkbfile-devel,\
+zlib,\
+font-util,\
+khronos-opengl-registry,\
+python3-lxml,\
+xorg-util-macros,\
+xtrans,\
+xkbcomp,\
+xkeyboard-config,\
+libnettle-devel,\
+libepoxy-devel,\
+libtirpc-devel"
 - echo Install done
 cache:
 - C:\cache