examples: fix shellcheck in build scripts

examples/common/build_ice40.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 set -euo pipefail
+
+# shellcheck source=examples/common/find_tool.sh
 source "$(dirname "$(readlink -f "$0")")/find_tool.sh"
 
 # Make sure tools are accessible

examples/common/build_vivado.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 set -euo pipefail
+
+# shellcheck source=examples/common/find_tool.sh
 source "$(dirname "$(readlink -f "$0")")/find_tool.sh"
 
 # Make sure Vivado is accessible

examples/common/find_tool.sh

@@ -1,3 +1,5 @@
+#!/usr/bin/env bash
+
 # Find a tool binary from either the $PATH environment variable, or another
 # environment variable named after the tool (ie, $VIVADO or $YOSYS).
 # Usage: find_tool <tool_name>