Replace bash-specific pushd/popd with POSIX cd in devcontainer script

[MiracleInvoker]

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Reference Issues/PRs

None

What does this implement/fix? Explain your changes.

This PR fixes a failure in the Dev Container setup script caused by the use of pushd and popd commands in a script running under #!/bin/sh.

The Problem:
The .devcontainer/postCreateCommand.sh script uses pushd and popd, which are Bash extensions and not defined in the POSIX sh standard. This causes the script to fail on systems where /bin/sh is a strict POSIX shell (like dash), preventing the environment from building.

The Fix:
Replaced pushd and popd with standard cd commands. This allows the script to run successfully using the default #!/bin/sh interpreter without requiring a dependency on bash.