Convert variables to `str` before replacing values in templates

[MLBZ521]

When a substitutable key with non-str (e.g. boolean-like) value is ran through xml.sax.saxutils.escape, it's being converted to a bool and then attempts to run str's .replace method on it, causing it to fail.

Sample error:

JamfPolicyUploader: Replacing any instances of 'POLICY_TRIGGER_CHECKIN' with 'true'
Traceback (most recent call last):
  File "/Library/AutoPkg/autopkglib/__init__.py", line 840, in process
    self.env = processor.process()
  File "/Library/AutoPkg/autopkglib/__init__.py", line 626, in process
    self.main()
  File "/Library/<username>/AutoPkg/RecipeRepos/com.github.autopkg.grahampugh-recipes/JamfUploaderProcessors/JamfPolicyUploader.py", line 341, in main
    template_xml = self.prepare_policy_template(
  File "/Library/<username>/AutoPkg/RecipeRepos/com.github.autopkg.grahampugh-recipes/JamfUploaderProcessors/JamfPolicyUploader.py", line 123, in prepare_policy_template
    template_contents = self.substitute_assignable_keys(
  File "/Library/<username>/AutoPkg/RecipeRepos/com.github.autopkg.grahampugh-recipes/JamfUploaderProcessors/JamfUploaderLib/JamfUploaderBase.py", line 634, in substitute_assignable_keys
    replacement_key = escape(self.env.get(found_key))
  File "/Library/AutoPkg/Python3/Python.framework/Versions/3.10/lib/python3.10/xml/sax/saxutils.py", line 27, in escape
    data = data.replace("&", "&amp;")
AttributeError: 'bool' object has no attribute 'replace'
  File "/Library/AutoPkg/autopkglib/__init__.py", line 840, in process
    self.env = processor.process()
'bool' object has no attribute 'replace'
Failed.

AutoPkg is currently written to convert all the YAML values to their Python types, which helps lead to this issue. I have submitted a proposal to prevent this in AutoPkg itself as well in #911, but not sure if this is something that will be/or should be considered as it makes an assumption that all variable types are and should be of the str type.

This is some-what the recipe writers fault, which I acknowledge as well.

[grahampugh]

Thanks, seems harmless. Probably not something they could or should consider in AutoPkg generally since there can be values that are, for example, dictionaries.

[MLBZ521]

Yeah, from JamfUploader, in this specific location, it shouldn't cause an issue -- I didn't think.

From a higher level, in autopkg, it may/should be more...targeted, is what I'm thinking as well.