Do not cancel in-progress checks on the main branch

See https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency

* Previously there was a single concurrency group per each branch.
  As the `main` branch got pushed into frequently, very few commits got
  tested to the end. It resulted in "broken" `main` branch as there were
  no fully successful workflow runs.
  Now the `main` branch gets a separate concurrency group for each commit.
* As GitHub Actions syntax does not have the conditional operator, it is
  emulated via logical and/or operations. Although undocumented, they
  return one of their operands instead of plain true/false.
* Replace 3-space indentation with 2-space indentation while we are here
  to be consistent with the rest of the file.

.github/workflows/build_and_test.yml

@@ -11,8 +11,9 @@ defaults:
     shell: bash -ex {0}
 
 concurrency:
-   group: ${{ github.workflow }}-${{ github.ref }}
-   cancel-in-progress: true
+  # Allow only one workflow per any non-`main` branch.
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref == 'refs/heads/main' && github.sha || 'anysha' }}
+  cancel-in-progress: true
 
 env:
   RUST_BACKTRACE: 1

.github/workflows/codestyle.yml

@@ -11,8 +11,9 @@ defaults:
     shell: bash -ex {0}
 
 concurrency:
-   group: ${{ github.workflow }}-${{ github.ref }}
-   cancel-in-progress: true
+  # Allow only one workflow per any non-`main` branch.
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref == 'refs/heads/main' && github.sha || 'anysha' }}
+  cancel-in-progress: true
 
 env:
   RUST_BACKTRACE: 1

.github/workflows/pg_clients.yml

@@ -13,8 +13,9 @@ on:
   workflow_dispatch:
 
 concurrency:
-   group: ${{ github.workflow }}-${{ github.ref }}
-   cancel-in-progress: true
+  # Allow only one workflow per any non-`main` branch.
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref == 'refs/heads/main' && github.sha || 'anysha' }}
+  cancel-in-progress: true
 
 jobs:
   test-postgres-client-libs: