write_aig_external doesn't work with filenames that include path separator characters

[weaversa]

sawscript> write_aig_external "../test.aig" {{ \x -> x + 0xa }}

Stack trace:
"write_aig_external" (<stdin>:1:1-1:19):
openTempFile': Template string must not contain path separator characters: ../test.aig.v

[atomb]

This should be an easy fix. We should create the temporary file based on just the filename portion of the output, not the whole path.