code coverage breakage

# Bug report

The commit `143bc9e40f7f33695216db60f73e10a900d1d1cd ("feat: Single process optimization (#5489)")` changed the following in the `cloud-init-local.service.tmpl`

```
-ExecStart=/usr/bin/cloud-init init --local
+# This service is a shim which preserves systemd ordering while allowing a
+# single Python process to run cloud-init's logic. This works by communicating
+# with the cloud-init process over a unix socket to tell the process that this
+# stage can start, and then wait on a return socket until the cloud-init
+# process has completed this stage. The output from the return socket is piped
+# into a shell so that the process can send a completion message (defaults to
+# "done", otherwise includes an error message) and an exit code to systemd.
+ExecStart=sh -c 'echo "start" | nc.openbsd -Uu -W1 /run/cloud-init/share/local.sock -s /run/cloud-init/share/local-return.sock | sh'
```
and for other unit files as well. Example, for `cloud-config.service.tmpl`

```
-ExecStart=/usr/bin/cloud-init modules --mode=config
+# This service is a shim which preserves systemd ordering while allowing a
+# single Python process to run cloud-init's logic. This works by communicating
+# with the cloud-init process over a unix socket to tell the process that this
+# stage can start, and then wait on a return socket until the cloud-init
+# process has completed this stage. The output from the return socket is piped
+# into a shell so that the process can send a completion message (defaults to
+# "done", otherwise includes an error message) and an exit code to systemd.
+ExecStart=sh -c 'echo "start" | nc.openbsd -Uu -W1 /run/cloud-init/share/config.sock -s /run/cloud-init/share/config-return.sock | sh'
```
The `enable_coverage.py` looks for `ExecStart=/usr` and replaces with a code that calls the code coverage tool:

```
content = content.replace(
"ExecStart=/usr",
(
"ExecStart=python3 -m coverage run "
"--source=/usr/lib/python3/dist-packages/cloudinit --append /usr"
),
)

```
so that the unit files when executed can start collecting the code coverage data. This is broken with the change.

## Steps to reproduce the problem
Try to run code coverage integration test in cloud-init. It does not work.

## Environment details
- Cloud-init version: latest
- Operating System Distribution: Ubuntu 24.4
- Cloud provider, platform or installer type: lxd container

## cloud-init logs
```
2026-02-26 12:16:08 WARNING integration_testing:conftest.py:569 Could not generate report during session finish: [Errno 2] No such file or directory: '/tmp/cloud_init_test_logs/260226121439'
...

Message: 'Could not generate report during session finish: %s'
Arguments: (FileNotFoundError(2, 'No such file or directory'),
```