fix: R CI docsite issue (deephaven#5729)

Author: alexpeters1208

R/build.gradle

@@ -179,7 +179,10 @@ def rClientSite = Docker.registerDockerTask(project, 'rClientSite') {
         runCommand("mkdir -p ${prefix}/src/rdeephaven/docs")
         runCommand('''echo "status = tryCatch(" \
                            "   {" \
-                           "      install.packages('pkgdown', repos='https://cran.r-project.org'); " \
+                           "      install.packages('devtools'); " \
+                           "      require(devtools); " \
+                           "      remove.packages('pkgdown'); " \
+                           "      devtools::install_version('pkgdown', version='2.0.0', repos='https://cran.r-project.org'); " \
                            "      0" \
                            "   }," \
                            "  error=function(e) 1," \