Install: fix postinstall script on osx

The macOS install encounteres the following error when running the postinstall script:

    ./postinstall: /tmp/PKInstallSandbox.qGkbqk/Scripts/com.microsoft.GitCredentialManager.kjGWk5/postinstall: line 21: [: missing `]'

This resulted in the credential helper not being correctly configured.

The problem is with the test on the indicated line - the '[ ]' test does not
support '&&' statements inside the test expression, as it splits the expression into separate
commands.

Author: jamill

src/osx/Installer.Mac/scripts/postinstall

@@ -18,7 +18,7 @@ function IsBrewPkgInstalled
 }
 
 # Check if Java GCM is present on this system and unlink it should it exist
-if [ -L /usr/local/bin/git-credential-manager && IsBrewPkgInstalled "git-credential-manager" ]
+if [ -L /usr/local/bin/git-credential-manager ] && IsBrewPkgInstalled "git-credential-manager";
 then
     brew unlink git-credential-manager
 fi