add merge_group trigger to pr-check-file-encodings.yml

[cjyetman]

Using the merge queue has not been working recently because the file encodings check seems to run forever but never complete. I believe this will fix that, because that action was not enabled for the merge queue.

[github-actions]

Coverage Report

file	head	main	diff
Overall	62%	62%	0%
R/canonize_path.R	91%	91%	0%
R/char_to_.R	0%	0%	0%
R/determine_header_types.R	92%	92%	0%
R/determine_headers.R	87%	87%	0%
R/get_csv_specs.R	0%	0%	0%
R/guess_delimiter.R	100%	100%	0%
R/guess_file_encoding.R	89%	89%	0%
R/guess_numerical_mark.R	94%	94%	0%
R/has_binary_null.R	100%	100%	0%
R/has_consistent_fields_per_line.R	100%	100%	0%
R/has_header.R	89%	89%	0%
R/has_newline_at_end.R	100%	100%	0%
R/is_file_accessible.R	100%	100%	0%
R/is_readable_file.R	100%	100%	0%
R/is_text_file.R	100%	100%	0%
R/is_valid_currency_code.R	100%	100%	0%
R/is_valid_cusip.R	100%	100%	0%
R/is_valid_isin.R	100%	100%	0%
R/read_first_line.R	88%	88%	0%
R/read_portfolio_csv.R	91%	91%	0%
R/simplify_if_one_col_df.R	100%	100%	0%