fix: fix double doxygen installation on ubuntu 18

Author: aminya

dist/setup_cpp.js

@@ -58,14 +58,15 @@ export async function setupDoxygen(version: string, setupDir: string, arch: stri
       let installationInfo: InstallationInfo
       if (version === "") {
         installationInfo = setupAptPack("doxygen", undefined)
-      }
-      try {
-        // doxygen on stable Ubuntu repositories is very old. So, we use get the binary from the website itself
-        installationInfo = await setupBin("doxygen", version, getDoxygenPackageInfo, setupDir, arch)
-        setupAptPack("libclang-cpp9")
-      } catch (err) {
-        notice(`Failed to download doxygen binary. ${err}. Falling back to apt-get.`)
-        installationInfo = setupAptPack("doxygen", undefined)
+      } else {
+        try {
+          // doxygen on stable Ubuntu repositories is very old. So, we use get the binary from the website itself
+          installationInfo = await setupBin("doxygen", version, getDoxygenPackageInfo, setupDir, arch)
+          setupAptPack("libclang-cpp9")
+        } catch (err) {
+          notice(`Failed to download doxygen binary. ${err}. Falling back to apt-get.`)
+          installationInfo = setupAptPack("doxygen", undefined)
+        }
       }
       await setupGraphviz(getVersion("graphviz", undefined), "", arch)
       return installationInfo