When the cleanup function gets called, there's a chance the Instnace
field isn't populated yet, so store the instance ID separately and wait
for it to be terminated in case it's present.
The error would produce the following trace:
```
goroutine 1 [running]:
...
main.(*OSBuildJobImpl).Run.func1()
osbuild/osbuild-composer/cmd/osbuild-worker/jobimpl-osbuild.go:404 +0xc5
panic({0x55e2a76a1e40?, 0x55e2a906d2f0?})
/usr/lib/golang/src/runtime/panic.go:920 +0x270
github.com/osbuild/osbuild-composer/internal/cloud/awscloud.(*AWS).deleteFleetIfExists(0xc000faa840, 0xc0012718c0)
osbuild/osbuild-composer/internal/cloud/awscloud/secure-instance.go:441 +0x175
github.com/osbuild/osbuild-composer/internal/cloud/awscloud.(*AWS).TerminateSecureInstance(0x55e2a90825e0?, 0x2?)
osbuild/osbuild-composer/internal/cloud/awscloud/secure-instance.go:192 +0x1d
github.com/osbuild/osbuild-composer/internal/cloud/awscloud.(*AWS).RunSecureInstance.func1()
osbuild/osbuild-composer/internal/cloud/awscloud/secure-instance.go:75 +0x69
github.com/osbuild/osbuild-composer/internal/cloud/awscloud.(*AWS).RunSecureInstance(0xc000faa840, {0xc000afeade, 0x10}, {0x0, 0x0}, {0x0, 0x0}, {0xc001120f30, 0x24})
osbuild/osbuild-composer/internal/cloud/awscloud/secure-instance.go:169 +0x12a7
...
```
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