testing/units: support python3 for currency units update

The path to python is hardcoded upstream. This patch replaces the hardcoded path with one based on the current location of the python3 binary.
7 jobs for !10500 with units in 11 minutes and 42 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
passed #168895
x86_64 docker-alpine
lint

00:03:03

 
  Build
failed #168900
aarch64 ci-build docker-alpine
build-aarch64

00:05:20

passed #168901
armv7 ci-build docker-alpine
build-armv7

00:04:50

failed #168899
ppc64le ci-build docker-alpine
build-ppc64le

00:08:36

failed #168898
s390x ci-build docker-alpine
build-s390x

00:04:13

failed #168897
x86 ci-build docker-alpine
build-x86

00:03:39

failed #168896
x86_64 ci-build docker-alpine
build-x86_64

00:03:23

 
Name Stage Failure
failed
build-ppc64le Build
No job log
failed
build-aarch64 Build
No job log
failed
build-s390x Build
No job log
failed
build-x86 Build
No job log
failed
build-x86_64 Build
No job log