ci: remove existing /usr/src for freebsd

Author: jiegec

.cirrus.yml

@@ -7,6 +7,7 @@ task:
   install_script: pkg install -y bash git autoconf automake libtool groff pkgconf
   kernel_src_script: |
     uname -r
+    rm -rf /usr/src
     if [[ $(uname -r) == "14.3-RELEASE" ]]; then
       git clone --branch releng/14.3 https://git.FreeBSD.org/src.git /usr/src
     else