Fix end-to-end test failures on Raspberry Pi (#7349)

A recent change updated the some logic in the end-to-end tests to match the main branch, but an important detail was missed. The result is that end-to-end tests fail on Raspberry Pi in the release/1.4 branch because the tests don't recognize the device's operating system. This change corrects that problem.

I confirmed that the tests pass on Raspberry Pi with this change.

## Azure IoT Edge PR checklist:

test/Microsoft.Azure.Devices.Edge.Test.Common/linux/EdgeDaemon.cs

@@ -38,6 +38,7 @@ public static async Task<EdgeDaemon> CreateAsync(Option<string> packagesPath, Ca
             // Trim potential whitespaces and double quotes
             char[] trimChr = { ' ', '"' };
             os = os.Split('=').Last().Trim(trimChr).ToLower();
+            os = os == "raspbian" ? "debian" : os;
             // Split potential version description (in case VERSION_ID was not available, the VERSION line can contain e.g. '7 (Core)')
             version = version.Split('=').Last().Split(' ').First().Trim(trimChr);