Check if the copy file source is absolute

[sudhackar]

From the mm discussion
https://github.com/canonical/ubuntu-image/blob/main/internal/imagedefinition/README.rst

    copy-file: (optional)
      -
        # The path to the file to copy.
        # The given path will be interpreted as relative to the
        # path of the image definition file if is not absolute.
        source: <string>

mentions

if is not absolute

but the code actually never checks that the path is absolute or not and always use it relative
from :

ubuntu-image/internal/statemachine/helper.go

Lines 756 to 758 in 4842e19

	source := filepath.Join(confDefPath, c.Source)
	dest := filepath.Join(targetDir, c.Dest)
	if debug {

Simple PR that fixes this.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.77%. Comparing base (bb3a3fe) to head (0830a68).
Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/statemachine/helper.go	25.00%	2 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (bb3a3fe) and HEAD (0830a68). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (bb3a3fe)	HEAD (0830a68)
unittests	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #261       +/-   ##
===========================================
- Coverage   93.96%   83.77%   -10.20%     
===========================================
  Files          18       18               
  Lines        3429     4375      +946     
===========================================
+ Hits         3222     3665      +443     
- Misses        133      632      +499     
- Partials       74       78        +4     

Flag	Coverage Δ
unittests	83.77% <25.00%> (-10.20%)	⬇️

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