fix: correctly modify PATH in install-proxy-rhcos.sh

The installer script wants to update the PATH variable to include
$BIN_PATH in blobfuse-proxs's systemd unit by setting:

Environment="PATH=<BIN_PATH>:$PATH"

BIN_PATH is evaluated in the installer script, and $PATH is escaped, to
be evaluated when the service unit runs.
However, environment variables like "$PATH" are NOT evaluated
by systemd in Environment="...". Quoting the documentation:

" Variable expansion is not performed inside the strings
  and the "$" character has no special meaning. "

https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#Environment

This effectively removes all other paths and sets PATH to ONLY <BIN_PATH>.
The resulting environment variable PATH contains the actual contents of
BIN_PATH and a literal '$PATH'.

This commit fixes that by wrapping ExecStart in bash -c '...' and
updating PATH there.

Author: t-lo

pkg/blobfuse-proxy/install-proxy-rhcos.sh

@@ -93,8 +93,9 @@ if [ "${INSTALL_BLOBFUSE_PROXY}" = "true" ];then
   echo "change from /usr/bin/blobfuse-proxy to ${BIN_PATH}/blobfuse-proxy in blobfuse-proxy.service"
   sed -i "s|/usr/bin/blobfuse-proxy|${BIN_PATH}/blobfuse-proxy|g" /blobfuse-proxy/blobfuse-proxy.service
   if [ "${BIN_PATH}" != "/usr/local/bin" ]; then
-    echo "add Environment=\"PATH=${BIN_PATH}:$PATH\" in blobfuse-proxy.service"
-    sed -i "/Delegate=yes/a Environment=\"PATH=${BIN_PATH}:\$PATH\"" /blobfuse-proxy/blobfuse-proxy.service
+    echo "add \"PATH=${BIN_PATH}:\$PATH\" in blobfuse-proxy.service ExecStart."
+    sed "s,^ExecStart[[:space:]]*=\\(.*\\)\$,ExecStart=/usr/bin/bash -c \"PATH=${BIN_PATH}:\$PATH \\1\"," \
+      /blobfuse-proxy/blobfuse-proxy.service
   fi
   if [ -f "/host/etc/systemd/system/blobfuse-proxy.service" ];then
     old=$(sha256sum /host/etc/systemd/system/blobfuse-proxy.service | awk '{print $1}')