Work-around for Cargo download dependency failed

This is a work-around for charm build failures resulting in:
"Cargo download dependency failed "send: no filter connected".

This is documented in the following upstream issue:
rust-lang/cargo#12202

This affects lunar builds where the curl version is 7.88.1.

Thanks to Colin Watson for help with figuring this out.

Closes-Bug: #2037589
Change-Id: I8d48de1e7f7c9f517f6759139610b2451c131c80

charmcraft.yaml

@@ -23,6 +23,7 @@ parts:
     build-environment:
       - CHARM_INTERFACES_DIR: $CRAFT_PROJECT_DIR/interfaces/
       - CHARM_LAYERS_DIR: $CRAFT_PROJECT_DIR/layers/
+      - CARGO_HTTP_MULTIPLEXING: "false"
 bases:
   - build-on:
       - name: ubuntu