deb: fix Debian package build script

The Debian package build script incorrectly copies the containing
directory to the destination tree rather than just the contents
recursively. This means we end up with:

usr
└── local
    ├── bin
    │   ├── Atlassian.Bitbucket.UI -> ../share/gcm-core/Atlassian.Bitbucket.UI
    │   ├── git-credential-manager-core -> ../share/gcm-core/git-credential-manager-core
    │   └── GitHub.UI -> ../share/gcm-core/GitHub.UI
    └── share
        └── gcm-core
 ** --->    └── Release
                ├── Atlassian.Bitbucket.UI
                ├── git-credential-manager-core
                ├── GitHub.UI
                ├── libHarfBuzzSharp.so
                ├── libSkiaSharp.so
                └── NOTICE

..rather than:

usr
└── local
    ├── bin
    │   ├── Atlassian.Bitbucket.UI -> ../share/gcm-core/Atlassian.Bitbucket.UI
    │   ├── git-credential-manager-core -> ../share/gcm-core/git-credential-manager-core
    │   └── GitHub.UI -> ../share/gcm-core/GitHub.UI
    └── share
        └── gcm-core
            ├── Atlassian.Bitbucket.UI
            ├── git-credential-manager-core
            ├── GitHub.UI
            ├── libHarfBuzzSharp.so
            ├── libSkiaSharp.so
            └── NOTICE

Since the symlinks are wrong, the installation is broken.

Author: mjcheetham

src/linux/Packaging.Linux/build.sh

@@ -174,7 +174,7 @@ Description: Cross Platform Git Credential Manager Core command line utility.
 EOF
 
 # Copy all binaries and shared libraries to target installation location
-cp -R "$PAYLOAD" "$INSTALL_TO" || exit 1
+cp -R "$PAYLOAD"/* "$INSTALL_TO" || exit 1
 
 # Create symlinks
 ln -s -r "$INSTALL_TO/git-credential-manager-core" \