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I'm working on a MIDI2CV board.
The schematic I'm working on is provided by the website of Kevin's Blog.
The MIDI cable comes the MIDI out of a Digitakt. It has 5V on pin5.
I was wondering if it is ok to connect the pin5 net to the global 5V of the circuit to avoid powering with external 5V like that.
No. For many reasons. It's just wrong, against MIDI standard, breaks isolation which allows for ground loops and would not even provide much power. If you say but other people use it too, well, they should not as technically that device does not comply with MIDI standard and therefore it will not be a MIDI device, and is not expected to work with actual MIDI devices.
First of all, the pins on connector are not called MIDI IN or MIDI out.
And there is usually no 5V on pin 5. Most often it is an open drain pin. Very rarely a MIDI output might use a TTL or CMOS output which pulls the pin high. And it is not intended to draw current from this pin.
There is 5V on pin 4, but pin 4 is connected to 5V ia 220 ohm resistor, not directly. So it can't be used for much power. It is used to provide a current source for the optoisolator LED.
And if you did use it for power, which it shouldn't, you would have to use pin 2 as ground. But this also means that the optoisolator becomes rather useless for isolation.
So even if it could be used, there is not enough power to do anything useful with it, and voltage will drop during data transmission.
The digital pin idles at 5V, rather than this being a connection to a 5V supply that you can use. 
Image from https://learn.sparkfun.com/tutorials/midi-tutorial/hardware--electronic-implementation
I've also seen diagrams showing pin 4 also connected via pullup to 5V, but not a direct connection. You can't use this to power your device.
