[Fix] `latest-npm`: ensure npm 7 is not installed on node < 10

nvm.sh

@@ -243,6 +243,12 @@ nvm_install_latest_npm() {
       fi
     fi
 
+    local NVM_IS_10_OR_ABOVE
+    NVM_IS_10_OR_ABOVE=0
+    if [ $NVM_IS_9_3_OR_ABOVE -eq 1 ] && nvm_version_greater_than_or_equal_to "${NODE_VERSION}" 10.0.0; then
+      NVM_IS_10_OR_ABOVE=1
+    fi
+
     if [ $NVM_IS_4_4_OR_BELOW -eq 1 ] || {
       [ $NVM_IS_5_OR_ABOVE -eq 1 ] && nvm_version_greater 5.10.0 "${NODE_VERSION}"; \
     }; then
@@ -260,6 +266,9 @@ nvm_install_latest_npm() {
     ; then
       nvm_echo '* `npm` `v6.9` is the last version that works on `node` `v6.0.x`, `v6.1.x`, `v9.0.x`, `v9.1.x`, or `v9.2.x`'
       $NVM_NPM_CMD install -g npm@6.9
+    elif [ $NVM_IS_10_OR_ABOVE -eq 0 ]; then
+      nvm_echo '* `npm` `v6.x` is the last version that works on `node` below `v10.0.0`'
+      $NVM_NPM_CMD install -g npm@6
     else
       nvm_echo '* Installing latest `npm`; if this does not work on your node version, please report a bug!'
       $NVM_NPM_CMD install -g npm