Expose sc_get_env() to read environment variable

Contrary to getenv(), sc_get_env() returns an allocated string that is
guaranteed to be encoded in UTF-8 on all platforms (it uses _wgetenv()
internally on Windows and converts the strings).

PR #5560 <https://github.com/Genymobile/scrcpy/pull/5560>

app/src/util/env.h

@@ -0,0 +1,12 @@
+#ifndef SC_ENV_H
+#define SC_ENV_H
+
+#include "common.h"
+
+// Return the value of the environment variable (may be NULL).
+//
+// The returned value must be freed by the caller.
+char *
+sc_get_env(const char *varname);
+
+#endif