bug: linkify with entities inside anchor strings are incorrectly escaped

[mcleanmds]

Describe the bug

linkify on a string with entities inside anchor element text results in the & character of the entity being incorrect escaped to &
e.g.   ->  

** python and bleach versions (please complete the following information):**

• Python Version: 3.9.5
• Bleach Version: 6.0.0

To Reproduce

A simple test to reproduce the behavior:

>>> from bleach import linkify
text = r'<p><a href="/">Some&nbsp;entity&rsquo;s</a>More&nbsp;entity&rsquo;s</p>'
expected = r'<p><a href="/" rel="nofollow">Some&nbsp;entity&rsquo;s</a>More&nbsp;entity&rsquo;s</p>'
assert linkify(text) == expected 

Expected behavior

linkify(r'<a href="/">Some&nbsp;entity&rsquo;s</a>')
'<a href="/" rel="nofollow">Some&nbsp;entity&rsquo;s</a>'

Actual behavior

linkify(r'<a href="/">Some&nbsp;entity&rsquo;s</a>')
'<a href="/" rel="nofollow">Some&amp;nbsp;entity&amp;rsquo;s</a>'

Additional context

This bug was introduced in 6.0.0 with the fix for #501 and #692: #692

[willkg]

Thank you for the issue!