Fix breakage due to https://github.com/rust-lang/cargo/pull/7560 and rust-lang/rust#66748

ci/build.sh

@@ -15,7 +15,7 @@ RUST=${TOOLCHAIN}
 echo "Testing Rust ${RUST} on ${OS}"
 
 if [ "${TOOLCHAIN}" = "nightly" ] ; then
-    cargo +nightly install cargo-xbuild -Z install-upgrade
+    cargo +nightly install cargo-xbuild
     rustup component add rust-src
 fi
 

ci/dox.sh

@@ -19,7 +19,7 @@ if ! rustc --version | grep -E "nightly" ; then
 fi
 
 rustup component add rust-src
-cargo +nightly install cargo-xbuild -Z install-upgrade
+cargo +nightly install cargo-xbuild
 
 # List all targets that do currently build successfully:
 # shellcheck disable=SC1003

ci/semver.sh

@@ -13,7 +13,7 @@ if ! rustc --version | grep -E "nightly" ; then
     exit 1
 fi
 
-cargo +nightly install semverver -Z install-upgrade
+cargo +nightly install semverver
 
 TARGETS=
 case "${OS}" in