ci: fix central mbt build

[1084565]

For the mbt build to work npm install needs to be called in the before-all section. Without that the npx can not be called and the mbt fails.

Without this fix the build fails in all clean (ci) environments without prior npm install.

From the build logs:

info  mtaBuild - Executing mta build call: "mbt build --mtar capire.sflight.mtar --platform CF --source ./ --target /home/jenkins/agent/workspace/abc"

leads to

error mtaBuild - [2024-11-28 12:21:00] ERROR the "before-all"" build failed: could not execute the "npx -p @sap/cds-dk cds build --profile production,node" command: exit status 1
info  mtaBuild - make: *** [Makefile_20241128122054.mta:28: pre_build] Error 1