nixos/test-driver: fix missing shellopts in `execute`

Without this fix, setting the shellopts in `machine.execute` is
inconsitent.  When no timeout is used, shellopts `set -euo pipefail` are
applied to the command as expected. When a timeout is specified, the
shellopts are not applied to the command itself (which is called inside
a `sh -c` that doesn't inherit the shellopts) but rather to the
`timeout` command, leading to the following full command:

```bash
(set -euo pipefail; timeout 900 sh -c 'cmd') | (base64 --wrap 0; echo)\n
```

With this fix, this is the command we get:

```bash
timeout 900 sh -c 'set -euo pipefail; false | true') | (base64 --wrap 0; echo)\n
```

0 files changed, 0 insertions, 0 deletions