chore(common): work around nvm being a function on linux, mac

On Linux, macOS, nvm is only provided as a bash function, which means it
is not available to the script unless nvm.sh is `source`d.
Unfortunately, this is fragile -- variable usage, function names, etc.
can all interfere with each other between build.sh and nvm.sh (for
example, the `$VERSION` variable).

So we have a wrapper script on those platforms to do our nvm invocation.

Relates-to: nvm-sh/nvm#3257

resources/build/_builder_nvm.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+
+set -e
+set -u
+
+if [[ $# -lt 1 ]]; then
+  echo "This script is called by shellHelperFunctions.sh, _select_node_version_with_nvm()"
+  echo "during build.sh configure steps. It is not intended to be called directly, as it"
+  echo "is a wrapper for nvm."
+  exit 99
+fi
+
+REQUIRED_NODE_VERSION="$1"
+
+#
+# nvm on macos and linux is a shell function. `source`ing it into our scripts is
+# fragile because it uses some of the same variable names that we do. Safest way
+# to work around this is to load it in a clean subshell and run it there.
+#
+# See also https://github.com/nvm-sh/nvm/issues/3257 (among others)
+#
+
+type -t nvm >/dev/null || {
+  source "$NVM_DIR/nvm.sh"
+  type -t nvm >/dev/null || {
+    echo "Failed to find nvm"
+    exit 1
+  }
+}
+
+nvm install "$REQUIRED_NODE_VERSION"
+nvm use "$REQUIRED_NODE_VERSION"

resources/shellHelperFunctions.sh

@@ -266,28 +266,22 @@ verify_npm_setup() {
 # Use nvm to select a node version according to package.json
 # This is typically run only on CI environments
 _select_node_version_with_nvm() {
-  type -t nvm >/dev/null || {
-    if [[ $BUILDER_OS != win ]]; then
-      # nvm on linux and macos is a shell function; we need to load it
-      source "$NVM_DIR/nvm.sh"
-    fi
-    type -t nvm >/dev/null || {
-      builder_die "Build environment setup error detected: nvm could not be found"
-    }
-  }
-
   local REQUIRED_NODE_VERSION="$("$JQ" -r '.engines.node' "$KEYMAN_ROOT/package.json")"
 
-  try_multiple_times nvm install "$REQUIRED_NODE_VERSION"
-  nvm use "$REQUIRED_NODE_VERSION"
+  if [[ $BUILDER_OS != win ]]; then
+    # launch nvm in a clean subshell, see _builder_nvm.sh for details
+    env -i bash "$KEYMAN_ROOT/resources/build/_builder_nvm.sh" "$REQUIRED_NODE_VERSION"
+  else
+    nvm install "$REQUIRED_NODE_VERSION"
+    nvm use "$REQUIRED_NODE_VERSION"
+  fi
 
   local CURRENT_NODE_VERSION="$(node --version)"
 
   # on windows, nvm use and nvm install always return success
   # https://github.com/coreybutler/nvm-windows/issues/738
   # note the 'v' prefix that node emits (and npm doesn't!)
   if [[ "$CURRENT_NODE_VERSION" != "v$REQUIRED_NODE_VERSION" ]]; then
-    #
     builder_die "Attempted to select node.js version $REQUIRED_NODE_VERSION but found $CURRENT_NODE_VERSION instead"
   fi
 }