Use python isolate mode

This (more or less) avoids user's environment variables tampering the
search process.
And also remove usages of `eval()` and `exec()`.

PR #18995.

src/base/utils/foreignapps.h

@@ -31,10 +31,13 @@
 
 #include <QString>
 
+#include "base/global.h"
 #include "base/utils/version.h"
 
 namespace Utils::ForeignApps
 {
+    inline const QString PYTHON_ISOLATE_MODE_FLAG = u"-I"_qs;
+
     struct PythonInfo
     {
         using Version = Utils::Version<3, 1>;