ci/install-extra-builddeps: Drop --root=/usr arg

AFAICT, we just need `cxxbridge` to be in the `PATH` of the building
user. Let's avoid targeting privileged paths so devs can just run this
script directly without `sudo`.

ci/install-extra-builddeps.sh

@@ -3,5 +3,5 @@
 set -xeuo pipefail
 if ! command -v cxxbridge; then
     ver=$(cargo metadata --format-version 1 | jq -r '.packages[]|select(.name == "cxx").version')
-    cargo install --root=/usr cxxbridge-cmd --version "${ver}"
+    cargo install cxxbridge-cmd --version "${ver}"
 fi