tools: add workaround for cpack problem with Cygwin

tools: add workaround for cpack problem with Cygwin

Copy the gml2gv executable to to the gv2gml alias instead of
symlinking it. Creating a symlink to gml2gv works fine in itself, but
results in an error like this when running cpack:

CPack Error: Problem while adding file </home/magja/graphviz/build/_CPack_Packages/CYGWIN/ZIP/Graphviz-2.49.1~dev.20210902.2049-CYGWIN/bin/gv2gml> to archive </home/magja/graphviz/build/_CPack_Packages/CYGWIN/ZIP/Graphviz-2.49.1~dev.20210902.2049-CYGWIN.zip>, ERROR = Unable to read from file 'Graphviz-2.49.1~dev.20210902.2049-CYGWIN/bin/gv2gml': Couldn't list extended attributes

gv2gml is a symlink. Trying to read the extended attributes manually gives:

attr -l /cygdrive/c/Users/magja/graphviz/build/_CPack_Packages/CYGWIN/ZIP/Graphviz-2.49.1~dev.20210902.2049-CYGWIN/bin/gv2gml
attr_list: Permission denied
Could not list /cygdrive/c/Users/magja/graphviz/build/_CPack_Packages/CYGWIN/ZIP/Graphviz-2.49.1~dev.20210902.2049-CYGWIN/bin/gv2gml