fix: CI Elixir shadowing and drop Zig 0.15.2 matrix

On macOS, `brew install elixir xz` installed Elixir 1.20.2 (OTP 29)
which shadowed setup-beam's Elixir 1.18.3 (OTP 27), causing a Hex ABI
mismatch (`beam_load.c` errors) at `mix deps.get`. Only `xz` is needed
via brew since setup-beam already provides Elixir.

Drop the 0.15.2 Zig matrix entries since lib/burrito.ex hardcodes
`@zig_version_expected` to 0.16.0 with a strict equality check, making
0.15.2 builds fail the version guard.

Author: gilbertwong96

.github/workflows/burrito-xcomp-check.yaml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         host: [ubuntu-latest, macos-latest]
-        zig: ["0.15.2", "0.16.0"]
+        zig: ["0.16.0"]
     steps:
       # Deps
       - uses: actions/checkout@v4
@@ -42,7 +42,7 @@ jobs:
       - run: sudo apt-get -y install xz-utils
         if: matrix.host == 'ubuntu-latest'
 
-      - run: brew install elixir xz
+      - run: brew install xz
         if: matrix.host == 'macos-latest'
 
       # Restore Cache
@@ -96,7 +96,7 @@ jobs:
       fail-fast: false
       matrix:
         host: [ubuntu-latest, macos-latest]
-        zig: ["0.15.2", "0.16.0"]
+        zig: ["0.16.0"]
     runs-on: windows-latest
     steps:
       - name: Download a single artifact
@@ -111,7 +111,7 @@ jobs:
       fail-fast: false
       matrix:
         host: [ubuntu-latest, macos-latest]
-        zig: ["0.15.2", "0.16.0"]
+        zig: ["0.16.0"]
     runs-on: ubuntu-latest
     steps:
       - name: Download a single artifact
@@ -127,7 +127,7 @@ jobs:
       fail-fast: false
       matrix:
         host: [ubuntu-latest, macos-latest]
-        zig: ["0.15.2", "0.16.0"]
+        zig: ["0.16.0"]
     runs-on: macos-latest
     steps:
       - name: Download a single artifact