installer: avoid running the UninstallRun entry more than once

Under certain circumstances, it is possible for `[UninstallRun]` entries
to be run multiple times. To avoid that, we now use a `RunOnceId`.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

src/windows/Installer.Windows/Setup.iss

@@ -100,7 +100,7 @@ Name: full; Description: "Full installation"; Flags: iscustom;
 Filename: "{app}\{#GcmExe}"; Parameters: "configure {#GcmConfigureCmdArgs}"; Flags: runhidden
 
 [UninstallRun]
-Filename: "{app}\{#GcmExe}"; Parameters: "unconfigure {#GcmConfigureCmdArgs}"; Flags: runhidden
+Filename: "{app}\{#GcmExe}"; Parameters: "unconfigure {#GcmConfigureCmdArgs}"; Flags: runhidden; RunOnceId: "unconfigure"
 
 [Files]
 Source: "{#PayloadDir}\*"; DestDir: "{app}"; Flags: ignoreversion recursesubdirs