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

This shell script will be used in environments (e.g. LAVA) where bash
is not available, hence let's make sure it is POSIX compliant in order
to be able to execute on any modern shell interpreter.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Guilherme Gallo <guilherme.gallo@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15208>

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

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 for var in \
     ASAN_OPTIONS \
@@ -106,8 +106,8 @@ for var in \
     VK_CPU \
     VK_DRIVER \
     VK_ICD_FILENAMES \
-    ; do
-  if [ -n "${!var+x}" ]; then
-    echo "export $var=${!var@Q}"
-  fi
+    ;
+do
+    eval val=\$${var}
+    [ -z "$val" ] || echo "export $var='$val'"
 done