Always update ccache symlinks in devel jobs.

[nuclearsandwich]

This should affect the build_and_test and build_and_install containers
of all Devel, PR, and CI jobs.

This change was tested locally by running

mkdir /tmp/nci && cd /tmp/nci && generate_ci_script.py https://raw.githubusercontent.com/ros-infrastructure/ros_buildfarm_config/production/index.yaml noetic bionic-python2 ubuntu bionic amd64 | sh

I didn't let the build complete because I needed my CPU back but I did observe many snippets like the following:

=> Processing catkin package: 'xmlrpcpp'                  
==> Building with env: '/tmp/ws/devel_isolated/test_roslib_comm/env.sh'
==> cmake /tmp/ws/src/ros_comm/utilities/xmlrpcpp -DCATKIN_DEVEL_PREFIX=/tmp/ws/devel_isolated/xmlrpcpp -DCMAKE_INSTALL_PREFIX=/tmp/ws/install_isolated -DBUILD_TESTING=1 -DCATKIN_ENABLE_TESTING=1 -DCATKIN_SKIP_TESTING=0 -DCATKIN_TEST_RES
ULTS_DIR=/tmp/ws/test_results -G Unix Makefiles in '/tmp/ws/build_isolated/xmlrpcpp'                                  
-- The C compiler identification is GNU 7.4.0              
-- The CXX compiler identification is GNU 7.4.0            
-- Check for working C compiler: /usr/lib/ccache/cc                                                                   
-- Check for working C compiler: /usr/lib/ccache/cc -- works
-- Detecting C compiler ABI info                           
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done                                                                                
-- Check for working CXX compiler: /usr/lib/ccache/c++
-- Check for working CXX compiler: /usr/lib/ccache/c++ -- works

suggesting that the ccache symlinks are being used.