Update buildx version 0.5.1 => 0.7.1

https://github.com/docker/buildx/releases/tag/v0.7.1
Signed-off-by: Davis Schirmer <djds@bghost.xyz>

hack/install-buildx

@@ -5,7 +5,7 @@ if [ -z "${BINDIR:-}" ] && [ -z "${PREFIX:-}" ]; then
   PREFIX="$(realpath "$(dirname "$0")/..")"
 fi
 
-: "${VERSION:=v0.5.1}"
+: "${VERSION:=v0.7.1}"
 : "${BINDIR:="${PREFIX}/bin"}"
 : "${DEST:="${BINDIR}/buildx"}"