Fix crash when deploing twice the same downloads:// url

See https://lkft.validation.linaro.org/scheduler/job/3983580

Job definition snippet:

```yaml
 - deploy:
    to: fastboot
    images:
      boot_a:
        url: downloads://lkft-x15-aosp-master-4.19-stable-lkft-boot.img
      boot_b:
        url: downloads://lkft-x15-aosp-master-4.19-stable-lkft-boot.img
```

Reported-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Rémi Duraffort <remi.duraffort@linaro.org>
18 jobs for fix-crash-downloads in 16 minutes and 26 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Test
passed dispatcher-debian-10 #141275
amd64

00:02:30

passed dispatcher-debian-11 #141277
amd64

00:02:30

passed server-debian-10 #141276
amd64

00:02:59

passed server-debian-11 #141278
amd64

00:05:06

 
  Analyze
passed bandit-sast #141279
amd64-dind

00:01:02

passed black #141282
amd64

00:00:53

passed code_quality #141283
amd64

00:00:29

passed codespell #141288
amd64

00:00:27

passed coverage #141284
amd64

00:06:59

61.02%
passed dockerfiles #141285
amd64

00:00:20

passed eslint-sast #141280
amd64-dind

00:00:32

passed pylint #141287
amd64

00:02:43

passed schemas #141286
amd64

00:00:40

passed semgrep-sast #141281
amd64-dind

00:01:16

 
  Build
passed debian/10 #141290
amd64

00:01:23

passed doc #141289
amd64

00:00:37

passed docker-amd64-dispatcher #141291
amd64-dind

00:03:44

passed docker-amd64-server #141292
amd64-dind

00:04:15