build: envkernel: Install makedepends before building package

See commit message.

Without the changes in this MR, you have to manually install the makedepends in the chroot before running pmbootstrap build --envkernel linux-postmarketos-qcom-msm8916.

(Until now I used a workaround because I was too lazy to install the makedepends manually: I ran pmbootstrap build --force linux-postmarketos-qcom-msm8916 to make it install the build dependencies, and interrupted it as soon as it started building the package... But this is annoying and you can easily build a broken package if you don't make sure all dependencies are installed...)

Ideally we would also respect the makedepends when initializing the build environment (i.e. when running source helpers/envkernel.sh). But this would require changing the usage of the script to make it aware of the kernel package that is going to be built. For now linux-postmarketos-qcom-msm8916 builds fine with the hard-coded dependencies, so we can postpone this until it actually starts causing problems for someone.