ci: pin actions by sha

[thaJeztah]

No description provided.

[kolyshkin]

To me this (using sha or defining a complete specific version) feels like a totally unnecessary complication which will create extra noise (more frequent updates, etc.).

Unless, of course,

• a specific action used tends to break often;
• or GHA is used for something very important and security-sensitive, like building an official release binary.

Neither seems to be the case here.