GitHub Workflows security hardening (#8267)

Signed-off-by: Alex <aleksandrosansan@gmail.com>

.github/workflows/r_tests.yml

@@ -6,6 +6,9 @@ env:
   R_PACKAGES: c('XML', 'data.table', 'ggplot2', 'DiagrammeR', 'Ckmeans.1d.dp', 'vcd', 'testthat', 'lintr', 'knitr', 'rmarkdown', 'e1071', 'cplm', 'devtools', 'float', 'titanic')
   GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
 
+permissions:
+  contents: read # to fetch code (actions/checkout)
+
 jobs:
   lintr:
     runs-on: ${{ matrix.config.os }}