Update azure-pipelines.yml for Azure Pipelines

azure-pipelines.yml

@@ -28,8 +28,8 @@ steps:
   displayName: 'Caching packages'
   continueOnError: true
 - bash: |
-    Rscript -e "install.packages(c('remotes', 'rcmdcheck'), lib=${R_LIBS_USER})"
-    Rscript -e "remotes::install_deps(dependencies=TRUE, lib=${R_LIBS_USER})"
+    Rscript -e "install.packages(c('remotes', 'rcmdcheck'), lib='${R_LIBS_USER}')"
+    Rscript -e "remotes::install_deps(dependencies=TRUE, lib='${R_LIBS_USER}')"
   displayName: 'Installing package dependencies'
 - bash: |
     Rscript -e "rcmdcheck::rcmdcheck(args = '--no-manual', error_on = 'error', check_dir = 'check')"