CI: fetch Proxifier from a source we control

raised in #1938

Author: belm0

.github/workflows/ci.yml

@@ -27,8 +27,8 @@ jobs:
             extra_name: ', with Komodia LSP'
           - python: '3.8'
             arch: 'x64'
-            lsp: 'https://www.proxifier.com/download/legacy/ProxifierSetup342.exe'
-            lsp_extract_file: ''
+            lsp: 'https://raw.githubusercontent.com/python-trio/trio-ci-assets/master/ProxifierSetup.zip'
+            lsp_extract_file: 'ProxifierSetup.exe'
             extra_name: ', with IFS LSP'
           #- python: '3.8'
           #  arch: 'x64'