Seems to fall over here:

~/hw/open_pdks $ git checkout 6cdeb7a2c5a90339512ee2f08948a8a5895626b8
HEAD is now at 6cdeb7a Updated version to go alonge with the last two pull request merges.
~/hw/open_pdks $ ./configure --with-sky130-source=$PDK_ROOT/skywater-pdk/libraries --with-sky130-local-path=$PDK_ROOT

resulting in..

~/hw/open_pdks $ ./configure --with-sky130-source=$PDK_ROOT/skywater-pdk/libraries --with-sky130-local-path=$PDK_ROOT
configure: Found technology directories: sky130
configure: Checking whether source path is specified for 'sky130'
readlink: illegal option -- f
usage: readlink [-n] [file ...]
configure: Checking specified path for 'sky130' at 
checking for ... no
configure: error: Specified path for 'sky130' at  not found

Made sure that

$PDK_ROOT

is configured before running

are you running on Mac? I believe I changed that to greadlink instead as work around.