fix: make CI-task "check for noisy stdout lines" more verbose. (#7868)

linter warnings print the following messages to stdout:

```
stdout:
./././MyProject/MyFile.lean:19:53: warning: unused variable `x` [linter.unusedVariables]
./././MyProject/MyFile.lean:19:53: warning: unused variable `y` [linter.unusedVariables]
```

With this change, the CI would at least print the first line after `stdout:`, partially helping with debugging.

.github/workflows/bors.yml

@@ -179,7 +179,7 @@ jobs:
 
       - name: check for noisy stdout lines
         run: |
-          ! grep "stdout:" stdout.log
+          ! grep --after-context=1 "stdout:" stdout.log
 
       - name: build library_search cache
         run: lake build -KCI MathlibExtras

.github/workflows/build.yml

@@ -185,7 +185,7 @@ jobs:
 
       - name: check for noisy stdout lines
         run: |
-          ! grep "stdout:" stdout.log
+          ! grep --after-context=1 "stdout:" stdout.log
 
       - name: build library_search cache
         run: lake build -KCI MathlibExtras

.github/workflows/build.yml.in

@@ -165,7 +165,7 @@ jobs:
 
       - name: check for noisy stdout lines
         run: |
-          ! grep "stdout:" stdout.log
+          ! grep --after-context=1 "stdout:" stdout.log
 
       - name: build library_search cache
         run: lake build -KCI MathlibExtras

.github/workflows/build_fork.yml

@@ -183,7 +183,7 @@ jobs:
 
       - name: check for noisy stdout lines
         run: |
-          ! grep "stdout:" stdout.log
+          ! grep --after-context=1 "stdout:" stdout.log
 
       - name: build library_search cache
         run: lake build -KCI MathlibExtras