[libcalamaresui] Branding uses $-substitution

Replace @{name} with ${name} to be consistent with the rest
of the replacement-code in Calamares.

src/branding/default/branding.desc

@@ -109,7 +109,7 @@ navigation: widget
 #
 # These strings support substitution from /etc/os-release
 # if KDE Frameworks 5.58 are available at build-time. When
-# enabled, @{var-name} is replaced by the equivalent value
+# enabled, ${varname} is replaced by the equivalent value
 # from os-release. All the supported var-names are in all-caps,
 # and are listed on the FreeDesktop.org site,
 #       https://www.freedesktop.org/software/systemd/man/os-release.html
@@ -121,7 +121,7 @@ navigation: widget
 # are visible as buttons there if the corresponding *show* keys
 # are set to "true" (they can also be overridden).
 strings:
-    productName:         "@{NAME}"
+    productName:         "${NAME}"
     shortProductName:    Generic
     version:             2020.2 LTS
     shortVersion:        2020.2

src/libcalamaresui/Branding.cpp

@@ -259,7 +259,7 @@ Branding::Branding( const QString& brandingFilePath, QObject* parent )
                 { QStringLiteral( "VARIANT_ID" ), relInfo.variantId() },
                 { QStringLiteral( "LOGO" ), relInfo.logo() } } };
             auto expand = [ & ]( const QString& s ) -> QString
-            { return KMacroExpander::expandMacros( s, relMap, QLatin1Char( '@' ) ); };
+            { return KMacroExpander::expandMacros( s, relMap, QLatin1Char( '$' ) ); };
 #else
             auto expand = []( const QString& s ) -> QString { return s; };
 #endif