Isar can't reuse sstate-cache build from different isar location

[Uladzimir Bely <ubely@ilbers.de>]

I tried to use the same SSTATE_DIR for different isar locations on the
same machine under the same user and found that it does not work.

Quick steps to reproduce it:

## Prepare build environment with two copies of isar:

```
~ $ mkdir -m 777 test-sstate
~ $ cd test-sstate/
~/test-sstate $ docker run --privileged -it -v .:/build
ghcr.io/siemens/kas/kas-isar:4.0
builder@ffe5274283d3:~$ cd /build
builder@ffe5274283d3:/build$ git clone
https://github.com/ilbers/isar.git isar-1
builder@ffe5274283d3:/build$ git clone
https://github.com/ilbers/isar.git isar-2
```

## Build isar-1:

```
builder@ffe5274283d3:/build$ cd /build/isar-1
builder@ffe5274283d3:/build/isar-1$ . isar-init-build-env 
builder@ffe5274283d3:/build/isar-1/build$ echo
'SSTATE_DIR="/build/sstate-cache"' >> conf/local.conf 
builder@ffe5274283d3:/build/isar-1/build$ echo
'DL_DIR="/build/downloads"' >> conf/local.conf
builder@ffe5274283d3:/build/isar-1/build$ time bitbake mc:qemuamd64-
bullseye:isar-image-base
real    4m38.759s
user    0m4.248s
sys     0m1.068s
builder@ffe5274283d3:/build/isar-1/build$ du -sh /build/downloads
229M    /build/downloads
builder@ffe5274283d3:/build/isar-1/build$ du -sh /build/sstate-cache
1.1G    /build/sstate-cache
```

## Build isar-2:

```
builder@ffe5274283d3:/build$ cd /build/isar-2
builder@ffe5274283d3:/build/isar-2$ . isar-init-build-env 
builder@ffe5274283d3:/build/isar-2/build$ echo
'SSTATE_DIR="/build/sstate-cache"' >> conf/local.conf 
builder@ffe5274283d3:/build/isar-2/build$ echo
'DL_DIR="/build/downloads"' >> conf/local.conf 
builder@ffe5274283d3:/build/isar-2/build$ time bitbake mc:qemuamd64-
bullseye:isar-image-base
real    4m32.314s
user    0m3.975s
sys     0m0.898s
builder@ffe5274283d3:/build/isar-2/build$ du -sh /build/downloads
229M    /build/downloads
builder@ffe5274283d3:/build/isar-2/build$ du -sh /build/sstate-cache/
2.2G    /build/sstate-cache/
```

Second build took almost the same time as first one. Sstate cache was
not reused, second build just added new files to the cache.

I tried similar approach with Yocto, and it works in it.

After debugging, I have concluded, that this issue comes from
"bootstrap" task that is not sstate-cache compatible. It's caused by
our manipulations with SRC_URI we do in it, so that it's different for
both builds. E.g.:

SRC_URI="file://locale file://chroot-setup.sh file:///build/isar-
1/meta/conf/distro/debian-bullseye.list"
SRC_URI="file://locale file://chroot-setup.sh file:///build/isar-
2/meta/conf/distro/debian-bullseye.list"

SRC_URI is a variable that is tracked in sstate-cache. Since the last
part (that we add "on fly" with get_aptsources_list() function) is
different for both builds, sstate-cache can't be reused for "isar-
bootstrap-<host,target>" and for all other tasks since they depend on
those ones.

Any ideas how to properly deal with this?

By the way, kas users are not affected, since fixed paths are used
(e.g., "/work/isar", "/build") under the container.