Change to not throw when a validator is not of type AttributeValidator

[skirchner989]

Found an issue adding validators via the builder that were not of the AttributeValidator type.

This is the error encountered:

System.InvalidCastException: Unable to cast object of type 'ArgumentMustBeValidEnvironmentContext' to type 'McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator'.
   at System.Linq.Enumerable.<CastIterator>d__97`1.MoveNext()

[codecov]

Codecov Report

Merging #431 (e691cc3) into main (1d6e1a5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #431   +/-   ##
=======================================
  Coverage   82.00%   82.00%           
=======================================
  Files         106      106           
  Lines        3284     3284           
=======================================
  Hits         2693     2693           
  Misses        591      591           

Impacted Files	Coverage Δ
...mandLineUtils/HelpText/DefaultHelpTextGenerator.cs	99.34% <100.00%> (ø)

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