fix(bundler): use relative symlinks for AppImage .DirIcon and .desktop

[alexrzem]

The symlink targets were created using app_dir_path.join() which produced absolute paths pointing to the build directory. When the AppImage is mounted at runtime, these symlinks are broken, preventing appimaged and file managers from extracting the application icon.

Fixes #15110
Confirms #15122 without using AI.

• cargo test passed all tests
• cargo clippy finished with no errors

[github-actions]

Package Changes Through b61658b

There are 7 changes which include tauri-macos-sign with patch, tauri-build with patch, tauri with minor, tauri-bundler with patch, tauri-cli with patch, @tauri-apps/cli with patch, tauri-utils with patch

Planned Package Versions

The following package releases are the planned based on the context of changes in this pull request.

package	current	next
tauri-utils	2.8.3	2.8.4
tauri-macos-sign	2.3.3	2.3.4
tauri-bundler	2.8.1	2.8.2
tauri-runtime	2.10.1	2.10.2
tauri-runtime-wry	2.10.1	2.10.2
tauri-codegen	2.5.5	2.5.6
tauri-macros	2.5.5	2.5.6
tauri-plugin	2.5.4	2.5.5
tauri-build	2.5.6	2.5.7
tauri	2.10.3	2.11.0
@tauri-apps/cli	2.10.1	2.10.2
tauri-cli	2.10.1	2.10.2

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