[CI] oldubuntu

Author: MikailBag

.github/workflows/rust.yml

@@ -57,7 +57,7 @@ jobs:
   tests:
     strategy:
       matrix:
-        os: ["ubuntu-20.04", "windows-2019"]
+        os: ["ubuntu-18.04", "windows-2019"]
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps:
@@ -67,7 +67,7 @@ jobs:
           toolchain: nightly
           override: true
       - name: Compile tests (Linux)
-        if: matrix.os == 'ubuntu-20.04'
+        if: matrix.os == 'ubuntu-18.04'
         run: |
           rm -r .cargo
           cd minion-tests
@@ -81,7 +81,7 @@ jobs:
           rustup target add x86_64-pc-windows-gnu
           cargo build -Zunstable-options --out-dir=../out --target=x86_64-pc-windows-gnu
       - name: Run tests (Linux)
-        if: matrix.os == 'ubuntu-20.04'
+        if: matrix.os == 'ubuntu-18.04'
         timeout-minutes: 3 
         run: |
           # temp