Add CFLAGS, LDFlAGS and PKG_CONFIG for building lxml

Author: mxamin

.github/workflows/linuxbrew.yml

@@ -45,6 +45,9 @@ jobs:
         run: |
           python3 -m venv test_venv
           source test_venv/bin/activate
+          export CFLAGS="-I$(brew --prefix)/include"
+          export LDFLAGS="-L$(brew --prefix)/lib"
+          export PKG_CONFIG_PATH="$(brew --prefix)/opt/libxml2/lib/pkgconfig"
           pip3 install --upgrade --no-binary=lxml -r requirements-test.txt
           pip3 install xmlsec --only-binary=xmlsec --no-index --find-links=dist/
           pytest -v --color=yes