`exec::finally()` shouldn't include `std::exception_ptr` in the error types if neither the source or final senders include that error type

I would have expected the following code to compile, but it fails because let_error tries to instantiate a call to the lambda with an argument of type std::exception_ptr.

https://godbolt.org/z/PK8YjohW8

#include <stdexec/execution.hpp>
#include <exec/finally.hpp>

#include <cstdio>

struct X {
    X() : value(0) { std::puts("X::X()"); }
    X(X&& o) noexcept : value(o.value) { std::puts("X::X(X&&)"); }
    X(const X& o) noexcept : value(o.value)  { std::puts("X::X(const X&)"); }
    ~X() { std::puts("X::~X()"); value = -1; }
    int value;
};

int main() {
    auto result = stdexec::sync_wait(
        stdexec::let_error(
            exec::finally(
                stdexec::just_error(X{}),
                stdexec::just()),
            [](const X& x) noexcept {
                std::printf("caught X(value=%i)\n", x.value);
                return stdexec::just();
            }));
}

I suspect that this is because we don't know ahead of time whether connect() is potentially throwing.
We could potentially work around this by instead connecting the final-sender at connect-time of the finally sender. This way, we know that we will be able start the final-operation when the input operation completes regardless of whether the connect() method is throwing or not.

Start with the reproducer linked in the issue and inspect exec/finally.hpp together with the sender and error-type handling in stdexec/execution.hpp. Trace why let_error is instantiated with std::exception_ptr when neither sender declares it. Done means the example compiles without that extra error type while preserving finally behavior.