ci: make sure liblzma is installed

[flavorjones]

What problem is this PR intended to solve?

Recently we've been getting upstream CI failures for libxml2 head on ubuntu: https://github.com/sparklemotion/nokogiri/actions/runs/12705536326/job/35416744559

configure:15841: checking for liblzma
configure:15848: $PKG_CONFIG --exists --print-errors "liblzma"
Package liblzma was not found in the pkg-config search path.
Perhaps you should add the directory containing `liblzma.pc'
to the PKG_CONFIG_PATH environment variable
Package 'liblzma', required by 'virtual:world', not found
configure:15851: $? = 1
configure:15865: $PKG_CONFIG --exists --print-errors "liblzma"
*** ../../../../ext/nokogiri/extconf.rb failed ***