Don't write shell env command to history

nRF Connect for VS Code

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nRF Connect for VS Code

To recreate the command line environment, the extension launches a terminal session as part of the startup procedure. Previously, this caused the extension to append echo ZEPHYR_BASE=${ZEPHYR_BASE} to the shell history file on startup. We have changed the approach for getting this environment, and the extension will no longer pollute the command line history.