Pin Czkawka to Rust 1.51.0 (#431)

Author: qarmin

.github/workflows/linux.yml

@@ -126,7 +126,7 @@ jobs:
   linux-gui:
     strategy:
       matrix:
-        toolchain: [ stable ]
+        toolchain: [ stable, 1.51.0 ]
         type: [ release ]
     runs-on: ubuntu-18.04
     steps:

instructions/Compilation.md

@@ -3,7 +3,7 @@
 ## Requirements
 Program  | Min  | What for
 ---------|------|------------------------------------------------------------
-Rust     | 1.51 | Czkawka, for now, aims to support only the latest stable Rust version
+Rust     | 1.51 | Czkawka, aims to support the latest available version of Rust on Ubuntu 20.04
 GTK      | 3.22 | Only for the `GTK` backend
 
 If you only want the terminal version without a GUI, just skip all the packages with `gtk` in their names.