[Bug][Build] Fix the CENTEC_SAI_DEV URL not found issue

[xumia]

Why I did it

Fix centec-arm64 build failure.
See https://dev.azure.com/mssonic/build/_build/results?buildId=117994&view=logs&j=88e1b2d1-110a-5bbf-c794-091704096bca

[  FAIL LOG END  ] [ target/debs/buster/libsai-dev_1.10.1-0_arm64.deb ]
make: *** [slave.mk:469: target/debs/buster/libsai-dev_1.10.1-0_arm64.deb] Error 1
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
make[1]: *** [Makefile.work:401: buster] Error 2
make[1]: Leaving directory '/data/myagent/_work/5/s'
make: *** [Makefile:32: target/sonic-centec-arm64.bin] Error 2

How I did it

Fix the URL variable definition issue in the Makefile.

How to verify it

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[guxianghong]

Thanks xuhui for fixing this bug. This bug is also fixed in PR #11186, but it haven't been reviewed.

[xumia]

Thanks xuhui for fixing this bug. This bug is also fixed in PR #11186, but it haven't been reviewed.

I merged your PR, closing my PR.