[actions] migrate Travis CI tests to GitHub Actions

Add new workflow for tests that were only running on Travis CI:
- fast test suite (sh, bash, dash, zsh)
- installation_iojs without curl (sh, bash, dash, zsh)
- installation_node with and without curl (sh, bash, dash, zsh)
- xenial test suite (sh, bash, dash, zsh)

The installation_node and xenial tests run on ubuntu-20.04 to match
the Travis xenial environment as closely as possible.

nvm.sh

@@ -2788,11 +2788,15 @@ nvm_die_on_prefix() {
   # npm first looks at $PREFIX (case-sensitive)
   # we do not bother to test the value here; if this env var is set, unset it to continue.
   # however, `npm exec` in npm v7.2+ sets $PREFIX; if set, inherit it
-  if [ -n "${PREFIX-}" ] && [ "$(nvm_version_path "$(node -v)")" != "${PREFIX}" ]; then
-    nvm deactivate >/dev/null 2>&1
-    nvm_err "nvm is not compatible with the \"PREFIX\" environment variable: currently set to \"${PREFIX}\""
-    nvm_err 'Run `unset PREFIX` to unset it.'
-    return 3
+  if [ -n "${PREFIX-}" ]; then
+    local NVM_NODE_VERSION
+    NVM_NODE_VERSION="$(node -v 2>/dev/null)"
+    if [ -z "${NVM_NODE_VERSION}" ] || [ "$(nvm_version_path "${NVM_NODE_VERSION}" 2>/dev/null)" != "${PREFIX}" ]; then
+      nvm deactivate >/dev/null 2>&1
+      nvm_err "nvm is not compatible with the \"PREFIX\" environment variable: currently set to \"${PREFIX}\""
+      nvm_err 'Run `unset PREFIX` to unset it.'
+      return 3
+    fi
   fi
 
   local NVM_OS
@@ -4624,24 +4628,24 @@ nvm_auto() {
         VERSION="$(nvm_resolve_local_alias default 2>/dev/null || nvm_echo)"
         if [ -n "${VERSION}" ]; then
           if [ "_${VERSION}" != '_N/A' ] && nvm_is_valid_version "${VERSION}"; then
-            nvm use --silent "${VERSION}" >/dev/null
+            nvm use --silent "${VERSION}" >/dev/null || :
           else
             return 0
           fi
         elif nvm_rc_version >/dev/null 2>&1; then
-          nvm use --silent >/dev/null
+          nvm use --silent >/dev/null || :
         fi
       else
-        nvm use --silent "${NVM_CURRENT}" >/dev/null
+        nvm use --silent "${NVM_CURRENT}" >/dev/null || :
       fi
     ;;
     install)
       local VERSION
       VERSION="$(nvm_alias default 2>/dev/null || nvm_echo)"
       if [ -n "${VERSION}" ] && [ "_${VERSION}" != '_N/A' ] && nvm_is_valid_version "${VERSION}"; then
-        nvm install "${VERSION}" >/dev/null
+        nvm install "${VERSION}" >/dev/null || :
       elif nvm_rc_version >/dev/null 2>&1; then
-        nvm install >/dev/null
+        nvm install >/dev/null || :
       else
         return 0
       fi