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GH-43548: [R][CI] Use grep -F to simplify matching or rchk output (#43477)

Passing along a minor change we made in our own mimeo of this GHA.

Resolves #43548
* GitHub Issue: #43548

Authored-by: Michael Chirico <chiricom@google.com>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>