Creating a stored procedure and adding date manually to filename
Declare a variable first - but use format 112 for this. Like this:

DECLARE @query_attachment_filename NVARCHAR(255) = 
    N'pca-test-' + 
    CONVERT(NCHAR(8), GETDATE(), 112) + 
    N'.csv';

EXEC msdb.dbo.sp_send_dbmail
@profile_name = 'support',
...
@attach_query_result_as_file = 1,
@query_attachment_filename = @query_attachment_filename,
@query_result_header =0,
@query_result_separator = ' '