scripts/snap_build.sh: clean before building

There's an issue with snap package generation after last merge,
but CI was failing with the wrong error message:

```
System.ArgumentException: Target directory already exists: /home/runner/work/geewallet/geewallet/staging/lib/geewallet
Parameter name: targetDir
  at FSI_0005.Fsdk.Misc.CopyDirectoryRecursively (System.IO.DirectoryInfo sourceDir, System.IO.DirectoryInfo targetDir,
```

This is because the snap package job is retried several times
(as per [1]), so we should clean up first before trying to
build, so that retries work.

[1] 30a7678

scripts/snap_build.sh

@@ -5,6 +5,9 @@ set -euxo pipefail
 # but as we're not using the 'make' plugin, we need to this manually now
 DEBIAN_FRONTEND=noninteractive sudo apt install -y fsharp build-essential pkg-config cli-common-dev mono-devel libgtk2.0-cil-dev
 
+# just in case this is a retry-run, we want to clean artifacts from previous try
+rm -rf ./staging
+
 ./configure.sh --prefix=./staging
 make
 make install