fix(ci/shellcheck): pass --shell=bash for per-file lint of board configs

The per-file ShellCheck loops in maintenance-lint-scripts.yml gate
invocation behind `grep -qE '^#!/.*bash' "$file"`, but grep scans every
line — so any board .conf with an embedded `#!/bin/bash` heredoc (e.g.
config/boards/easepi-a2.conf at lines 267/289/496) passes the gate.
The follow-up shellcheck call ran without --shell=bash, so it looked at
line 1 (a `# Rockchip ...` comment), couldn't determine the target
shell, and errored:

  In config/boards/easepi-a2.conf line 1:
  ^-- SC2148 (error): Tips depend on target shell and yours is unknown.
                      Add a shebang or a 'shell' directive.

Add --shell=bash to both call sites (findings artifact + red-cross
gate). ShellCheck now knows the shell up front, SC2148 no longer fires
on line 1, and the embedded heredocs inside the .conf still get linted
as bash.

Author: igorpecovnik

.github/workflows/maintenance-lint-scripts.yml

@@ -108,7 +108,7 @@ jobs:
             [[ -f "$file" ]] || continue
             if [[ ! "$file" =~ lib/|extensions/|\.py$|\.service$|\.rules$|\.network$|\.netdev$ ]]; then
               if grep -qE '^#!/.*bash' "$file" 2>/dev/null; then
-                "$SC" --severity=error --format=gcc "$file" >> shellcheck-findings.txt 2>&1 || true
+                "$SC" --shell=bash --severity=error --format=gcc "$file" >> shellcheck-findings.txt 2>&1 || true
               fi
             fi
           done
@@ -193,7 +193,7 @@ jobs:
             [[ -f "$file" ]] || continue
             if [[ ! "$file" =~ lib/|extensions/|\.py$|\.service$|\.rules$|\.network$|\.netdev$ ]]; then
               if grep -qE '^#!/.*bash' "$file" 2>/dev/null; then
-                shellcheck --severity=error "$file" || ret=$?
+                shellcheck --shell=bash --severity=error "$file" || ret=$?
               fi
             fi
           done