fix(afterPack): error on launch from deb terminal

When installing balena-etcher via apt on Debian/Ubuntu,
the command `balena-etcher-electron` fails with the error:
line 3: /usr/bin/balena-etcher-electron.bin: No such file or directory

This is because the /usr/bin/balena-etcher-electron is a symlink
to /opt/balenaEtcher/balena-etcher-electron, but the script looks
for balena-etcher-electron.bin in the symlink directory, not the
actual script location directory.

This commit uses `$(dirname "$(readlink -f "${BASH_SOURCE[0]}")")` to
find the real location of the balena-etcher-electron script without
symlink, so that balena-etcher-electron.bin is correctly found.

Change-Type: patch
Changelog-Entry: Fix error when launching from terminal when installed via apt.
Fixes: https://github.com/balena-io/etcher/issues/3074

afterPack.js

@@ -14,12 +14,16 @@ exports.default = function(context) {
   cp.execFileSync('mv', [scriptPath, binPath])
   fs.writeFileSync(
     scriptPath,
-    outdent`
+    outdent({trimTrailingNewline: false})`
       #!/bin/bash
+
+      # Resolve symlinks. Warning, readlink -f doesn't work on MacOS/BSD
+      script_dir="$(dirname "$(readlink -f "\${BASH_SOURCE[0]}")")"
+
       if [[ $EUID -ne 0 ]] || [[ $ELECTRON_RUN_AS_NODE ]]; then
-        "\${BASH_SOURCE%/*}"/${context.packager.executableName}.bin "$@"
+        "\${script_dir}"/${context.packager.executableName}.bin "$@"
       else
-        "\${BASH_SOURCE%/*}"/${context.packager.executableName}.bin "$@" --no-sandbox
+        "\${script_dir}"/${context.packager.executableName}.bin "$@" --no-sandbox
       fi
     `
   )