Update build instructions to reflect Zig dependency (#832)

.github/actions/setup-musl-gcc/action.yml

@@ -1,13 +0,0 @@
-name: "Setup musl-gcc"
-description: "Compile and install musl-gcc from source"
-
-runs:
-    using: "composite"
-    steps:
-        - run: |
-              git clone https://git.musl-libc.org/git/musl
-              cd musl
-              ./configure
-              make
-              sudo make install
-          shell: bash

BUILD.md

@@ -14,34 +14,25 @@ macOS does not have any platform-specific dependencies.
 
 #### Linux
 
-Make sure you have a recent install of GCC and Zip.
+You must have Zip installed. We also require the Zig compiler for statically linking CGO.
 
-Debian:
+Debian/Ubuntu:
 
 ```sh
-sudo apt install build-essential zip
+sudo apt install zip snapd
+sudo snap install zig --classic --beta
 ```
 
 Fedora:
 
 ```sh
-sudo dnf install make automake gcc gcc-c++ kernel-devel zip
+sudo dnf install zip zig
 ```
 
 Arch:
 
 ```sh
-sudo pacman -S base-devel zip
-```
-
-We also require `musl-gcc` for statically linking CGO. Run the following from wherever you keep your source code:
-
-```sh
-git clone https://git.musl-libc.org/git/musl
-cd musl
-./configure
-make
-sudo make install
+sudo pacman -S zip zig
 ```
 
 #### Windows