fix(ci): use own md5 tool for Bazel 9 compatibility

Bazel 9.0.0 flipped `--incompatible_strict_action_env` to `true` by
default, restricting `PATH` to `/bin:/usr/bin:/usr/local/bin`.

On macOS, this broke a few tests relying on the `md5sum` command:
```
Executing genrule //tests/deb:generate_md5sums failed: (Exit 127): [...]
[...]
/bin/bash: md5sum: command not found
```

This is because:
1. `md5sum` is not a native macOS command (macOS equivalent is `md5`),
2. Homebrew's `md5sum` is typically in `/opt/homebrew/bin`, which is no
   longer in the restricted `PATH`.

Since the repo already maintains a cross-platform `//tests/util:md5`
tool, the fix simply consists in leveraging it.

Author: rdesgroppes

tests/deb/BUILD

@@ -37,7 +37,8 @@ genrule(
     name = "generate_md5sums",
     srcs = [":generate_files"],
     outs = ["md5sums"],
-    cmd = "md5sum $(SRCS) | sed 's|$(RULEDIR)/||' > $@",
+    cmd = 'for f in $(SRCS); do echo "$$($(location //tests/util:md5) "$$f")  $${f#$(RULEDIR)/}"; done > $@',
+    tools = ["//tests/util:md5"],
 )
 
 my_package_naming(