`nvm debug`: Improve OS version detection

nvm.sh

@@ -2383,6 +2383,7 @@ nvm() {
 
     "debug" )
       local ZSH_HAS_SHWORDSPLIT_UNSET
+      local OS_VERSION
       ZSH_HAS_SHWORDSPLIT_UNSET=1
       if nvm_has "setopt"; then
         ZSH_HAS_SHWORDSPLIT_UNSET="$(set +e ; setopt | nvm_grep -q shwordsplit ; nvm_echo $?)"
@@ -2405,9 +2406,16 @@ nvm() {
       nvm_err "shell version: '$(${SHELL} --version | command head -n 1)'"
       nvm_err "uname -a: '$(command uname -a | command awk '{$2=""; print}' | command xargs)'"
       if [ "$(nvm_get_os)" = "darwin" ] && nvm_has sw_vers; then
-        nvm_err "OS version: $(sw_vers | command awk '{print $2}' | command xargs)"
+        OS_VERSION="$(sw_vers | command awk '{print $2}' | command xargs)"
       elif [ -r "/etc/issue" ]; then
-        nvm_err "OS version: $(command head -n 1 /etc/issue | command sed 's/\\.//g')"
+        OS_VERSION="$(command head -n 1 /etc/issue | command sed 's/\\.//g')"
+        if [ -z "${OS_VERSION}" ] && [ -r "/etc/os-release" ] ; then
+          # shellcheck disable=SC1091
+          OS_VERSION="$(. /etc/os-release && echo "${NAME}" "${VERSION}")"
+        fi
+      fi
+      if [ -n "${OS_VERSION}" ]; then
+        nvm_err "OS version: ${OS_VERSION}"
       fi
       if nvm_has "curl"; then
         nvm_err "curl: $(nvm_command_info curl), $(command curl -V | command head -n 1)"