ci: Fix FIH YAML result parsing

The running test was generating an YAML output with the following line:

```
last_line: '  Description: 'ECDSA signature test of attest token''
```

This string comes from the test with the single quotes, and using single
quotes twice break the YAML format, so this commit changes the string
after last_line to be enclosed in the double quotes.

Signed-off-by: Fabio Utzig <utzig@apache.org>