Revert "ci: Convert generate-env.sh to a POSIX compliant script"

This reverts commit 9904ea2c76 since it is
not able to properly escape all special characters (i.e. [']).

The POSIX conversion is not needed anymore, as we have made 'bash'
available in LAVA rootfs.

Reported-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15524>

.gitlab-ci/common/generate-env.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 for var in \
     ASAN_OPTIONS \
@@ -109,8 +109,8 @@ for var in \
     VK_CPU \
     VK_DRIVER \
     VK_ICD_FILENAMES \
-    ;
-do
-    eval val=\$${var}
-    [ -z "$val" ] || echo "export $var='$val'"
+    ; do
+  if [ -n "${!var+x}" ]; then
+    echo "export $var=${!var@Q}"
+  fi
 done