Add missing ICU library during R dependency installation

During the CI pipeline, the command

R -q -e 'remotes::install_deps(dependencies = TRUE)'

attempts to install required R packages, including stringr and its dependency stringi. The pipeline fails with this error message when building stringr:

Error in dyn.load(file, DLLpath = DLLpath, ...) :
unable to load shared object '/builds/HYCAR-Hydro/airgr/ci/lib/stringi/libs/stringi.so':
libicui18n.so.66: cannot open shared object file: No such file or directory
Calls: <Anonymous> ... namespaceImport -> loadNamespace -> library.dynam -> dyn.load
Execution halted
ERROR: lazy loading failed for package ‘stringr’

This error indicates that the required ICU system library (libicui18n.so.66) is missing from the build environment.

Proposed Solution:

Install the missing system dependency in advance:

apt-get update && apt-get install -y libicu-dev