aapt2: fix dangling reference to diag

The lambdas that ManifestFixer::BuildRules attach to XmlActionExecutor
outlive ManifestFixer::BuildRules, so they need to capture diag
by-value instead of by-reference.

Bug: 339119172
Test: aapt2_tests
Change-Id: I6b9b6a8733875247a5be12111413b45b99727e1f