configure.py: quote generated shell environment values

[moreaki]

configure.py writes non-Windows generated environment files as unquoted shell assignments, for example:

export PATH=/usr/bin:/Applications/Little Snitch.app/Contents/Components:/bin

When env.sh is sourced by bash, the space in the inherited PATH splits the assignment and makes the generated environment invalid. This can happen on macOS when applications install helper paths under .app bundles, but the issue is not macOS-specific: generated POSIX shell environment files should quote values before emitting export KEY=value.

Expected behavior:

export PATH='/usr/bin:/Applications/Little Snitch.app/Contents/Components:/bin'

Local validation used a generated test environment file with a path containing a space and verified that source restores the original PATH value exactly.