Fix timeout on systems with large numbers of loops

On systems with a very large number of snap packages installed, there
are a considerable number of loop devices. In this case, the `lsblk`
command in linuxdrivelist fills the stdout pipe, blocks, and the
rpi-imager process assumes it has timed out [1].

This is a trivial work-around that simply excludes loop devices
(major=7) from the `lsblk` output. Given subsequent code excludes
everything starting with `/dev/loop` anyway, there should be no change
in user experience with this exclusion.

[1]: https://github.com/waveform80/imager-snap/issues/6

src/linux/linuxdrivelist.cpp

@@ -48,7 +48,7 @@ namespace Drivelist
         std::vector<DeviceDescriptor> deviceList;
 
         QProcess p;
-        QStringList args = { "--bytes", "--json", "--paths", "--output-all" };
+        QStringList args = { "--bytes", "--json", "--paths", "--output-all", "--exclude", "7" };
         p.start("lsblk", args);
         p.waitForFinished(2000);
         QByteArray output = p.readAll();