Enable extensions in constant-time tests

[SWilson4]

Due to a configuration error, our weekly constant-time tests only run with OQS_OPT_TARGET=generic. This PR enables the intended configuration of OQS_OPT_TARGET=haswell by setting OQS_DIST_BUILD=OFF.

Fixes #1566.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in oqs-provider, OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)

[praveksharma]

Looks good to me!